Semiconductor device and method for manufacturing the same

ABSTRACT

It is an object to manufacture and provide a highly reliable display device including a thin film transistor with a high aperture ratio which has stable electric characteristics. In a manufacturing method of a semiconductor device having a thin film transistor in which a semiconductor layer including a channel formation region is formed using an oxide semiconductor film, a heat treatment for reducing moisture and the like which are impurities and for improving the purity of the oxide semiconductor film (a heat treatment for dehydration or dehydrogenation) is performed. Further, an aperture ratio is improved by forming a gate electrode layer, a source electrode layer, and a drain electrode layer using conductive films having light transmitting properties.

TECHNICAL FIELD

The present invention relates to a semiconductor device having a circuitformed using a thin film transistor (hereinafter referred to as TFT) anda manufacturing method thereof. For example, the present inventionrelates to electro-optical devices typified by liquid crystal displaypanels, or electronic devices which have light-emitting display devicescontaining an organic light-emitting element as a component.

Note that in this specification, a semiconductor device refers to alldevices that can function by utilizing semiconductor characteristics. Anelectro-optical device, a semiconductor circuit, and an electronicappliance are all semiconductor devices.

BACKGROUND ART

Note that as a method for providing a metal auxiliary wiring for atransparent electrode of an electro-optical element, a method by whichthe metal auxiliary wiring is provided so as to overlap with an uppersurface of the transparent electrode or a lower surface of thetransparent electrode and to be electrically connected to thetransparent electrode has been known (for example, see Patent Document1).

A structure in which an additional capacitor electrode provided for anactive matrix substrate is formed of a conductive film having a lighttransmitting property of ITO, SnO₂, or the like and an auxiliary wiringformed of a metal film is provided in contact with the additionalcapacitor electrode in order to reduce the electric resistance of theadditional capacitor electrode has been known (see Patent Document 2).

Note that it has been known that, as each of a gate electrode, a sourceelectrode, and a drain electrode of a field effect transistor formedusing an amorphous oxide semiconductor film, a transparent electrode ofindium tin oxide (ITO), indium zinc oxide, ZnO, SnO₂, or the like, ametal electrode of Al, Ag, Cr, Ni, Mo, Au, Ti, Ta, or the like, or ametal electrode of an alloy containing any of the above elements can beused; and, by stacking two or more of these layers, contact resistancemay be reduced or interface intensity may be improved (for example, seePatent Document 3).

Note that it has been known that, as a material of each of a sourceelectrode, a drain electrode, a gate electrode and an auxiliarycapacitor electrode of a transistor formed using an amorphous oxidesemiconductor, a metal such as indium (In), aluminum (Al), gold (Au), orsilver (Ag), or an oxide material such as indium oxide (In₂O₃), tinoxide (SnO₂), zinc oxide (ZnO), cadmium oxide (CdO), cadmium indiumoxide (CdIn₂O₄), cadmium tin oxide (Cd₂SnO₄), or zinc tin oxide(Zn₂SnO₄) can be used; and the same material or different materials maybe used for the gate electrode, the source electrode, and the drainelectrode (for example, see Patent Documents 4 and 5).

REFERENCE

[Patent Document 1] Japanese Published Patent Application No. H02-82221[Patent Document 2] Japanese Published Patent Application No. H02-310536

[Patent Document 3] Japanese Published Patent Application No.2008-243928 [Patent Document 4] Japanese Published Patent ApplicationNo. 2007-109918 [Patent Document 5] Japanese Published PatentApplication No. 2007-115807 DISCLOSURE OF INVENTION

However, since a conductive film having a light transmitting property isused just for an electrode material in a conventional display panelwhich uses an oxide semiconductor, an aperture ratio cannot be improved.In addition, when a display device is manufactured using metal oxide,its reliability has not been taken into consideration.

In view of the above, an object of one embodiment of the presentinvention is to improve both the aperture ratio and reliability of adisplay device formed using metal oxide.

In a manufacturing method of a semiconductor device having a thin filmtransistor in which a semiconductor layer including a channel formationregion is formed using an oxide semiconductor film, a heat treatment forreducing moisture and the like which are impurities and for improvingthe purity of the oxide semiconductor film (a heat treatment fordehydration or dehydrogenation) is performed. Further, impurities suchas moisture existing not only in the oxide semiconductor film but alsoin a gate insulating layer and at interfaces between the oxidesemiconductor film and a film above and in contact therewith and betweenthe oxide semiconductor film and a film below and in contact therewithare reduced.

One embodiment of the present invention disclosed in this specificationis a manufacturing method of a semiconductor device which includes thesteps of: forming a gate electrode layer including metal oxide over asubstrate having an insulating surface; forming a gate insulating layerover the gate electrode layer; forming an oxide semiconductor layer overthe gate insulating layer; dehydrating or dehydrogenating the oxidesemiconductor layer; forming a source electrode layer and a drainelectrode layer including metal oxide over the dehydrated ordehydrogenated oxide semiconductor layer; forming a protectiveinsulating layer in contact with part of the oxide semiconductor layer,over the gate insulating layer, the oxide semiconductor layer, thesource electrode layer, and the drain electrode layer; and forming apixel electrode layer including metal oxide over the protectiveinsulating layer.

For dehydration or dehydrogenation, a heat treatment is performed in anoxygen atmosphere, in an inert gas atmosphere such as nitrogen or a raregas (argon, helium, or the like), or under a reduced pressure at atemperature greater than or equal to 350° C. or preferably greater thanor equal to 400° C. and less than the strain point of the substrate,whereby an impurity such as moisture contained in the oxidesemiconductor layer is reduced.

Dehydration or dehydrogenation of the oxide semiconductor is conductedwith conditions of the heat treatment such that at least a peak ataround 300° C. of two peaks of water are/is not detected when thedehydrated or dehydrogenated oxide semiconductor layer is measured withthermal desorption spectroscopy (TDS) while the temperature is increasedto 450° C. Therefore, even when a thin film transistor using thedehydrated or dehydrogenated oxide semiconductor layer is subjected toTDS at a temperature as high as 450° C., at least a peak of water ataround 300° C. is not detected.

Then, slow cooling is performed from the heating temperature T at whichthe oxide semiconductor layer is dehydrated or dehydrogenated to atemperature low enough to prevent water from coming in again,specifically to a temperature which is more than 100° C. lower than theheating temperature T, or more preferably to a temperature less than orequal to 100° C.

A gas atmosphere in which the heating temperature T is decreased may beswitched to a gas atmosphere different from that in which thetemperature is increased to the heating temperature T.

Electric characteristics of a thin film transistor are improved and athin film transistor having mass productivity and high performance isrealized, by using an oxide semiconductor film which is formed byreducing moisture contained in the film by the heat treatment fordehydration or dehydrogenation and then subjected to slow cooling (orcooling) in an atmosphere containing no moisture (the dew pointtemperature of which is less than or equal to −40° C. or preferably,less than or equal to −60° C.).

In this specification, a heat treatment in an oxygen atmosphere, in aninert gas atmosphere such as nitrogen or a rare gas (argon, helium, orthe like), or under a reduced pressure is called a heat treatment fordehydration or dehydrogenation. For convenience, dehydration ordehydrogenation in this specification refers to not only elimination ofH₂ by a heat treatment but also elimination of H, OH, or the like by aheat treatment.

In the case where a heat treatment is performed in an inert gasatmosphere such as nitrogen or a rare gas (argon, helium, or the like)or under a reduced pressure, it can be said that: an oxide semiconductorlayer which has been of an i-type becomes an oxygen deficiency typelayer and has low resistance by the heat treatment, i.e., becomes ann-type (such as n⁻ or n⁺); and then, by forming an oxide insulating filmin contact with the oxide semiconductor layer, the oxide semiconductorlayer is placed into a state where oxygen is in excess so as to have ahigher resistance, i.e., becomes an i-type. Thus, a semiconductor deviceincluding a thin film transistor which has favorable electriccharacteristics and high reliability can be manufactured and provided.

In the case where the heat treatment is performed in an inert gasatmosphere such as nitrogen or a rare gas (argon, helium, or the like)or under a reduced pressure, and then the atmosphere is switched to anoxygen atmosphere so that slow cooling is performed, an oxidesemiconductor layer which has been of an i-type becomes an oxygendeficiency type layer and has low resistance by the heat treatment,i.e., becomes an n-type (such as n⁻ or n⁺), and then, the oxidesemiconductor layer is placed into a state where oxygen is in excess bythe slow cooling in the oxygen atmosphere so as to have a higherresistance, i.e., becomes an i-type.

In addition, in the case where the heat treatment for dehydration ordehydrogenation is performed in an oxygen atmosphere, moisture in theoxide semiconductor layer is released, whereby the oxide semiconductorlayer can be placed into a state where oxygen is in excess.

The term “oxide semiconductor” used in this specification is representedby InMO₃(ZnO)_(m) (m>0), and a thin film transistor in which the thinfilm of the oxide semiconductor is used as an oxide semiconductor layeris manufactured. Note that M represents one or more metal elementsselected from Ga, Fe, Ni, Mn, and Co. As an example, M may be Ga or mayinclude the above metal element in addition to Ga; for example, M may beGa and Ni or may be Ga and Fe. Moreover, in the above oxidesemiconductor, in some cases, a transition metal element such as Fe orNi or an oxide of the transition metal is contained as an impurityelement in addition to a metal element contained as M. In thisspecification, among the oxide semiconductor whose structures areexpressed by InMO₃ (ZnO)_(m) (m>0), an oxide semiconductor whichincludes Ga as M is referred to as an In—Ga—Zn—O-based oxidesemiconductor and a thin film of the In—Ga—Zn—O-based oxidesemiconductor is referred to as an In—Ga—Zn—O-based non-single-crystalfilm.

As the metal oxide applied to the oxide semiconductor layer, any of thefollowing metal oxide can be applied besides the above: anIn—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metal oxide; aSn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide; aSn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; aSn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-basedmetal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide.Alternatively, the silicon oxide may be included in the oxidesemiconductor layer formed using the above metal oxide.

The oxide semiconductor preferably includes In, or more preferably, Inand Ga. Dehydration or dehydrogenation is effective in changing theoxide semiconductor layer into an i-type (intrinsic).

In the case where the heat treatment for dehydration or dehydrogenationis performed after the oxide semiconductor layer is formed, the oxidesemiconductor layer which is amorphous is changed into amicrocrystalline film or a polycrystalline film in some cases dependingon the condition of the heat treatment or the material of the oxidesemiconductor layer. Further, the oxide semiconductor layer is partiallycrystallized in some cases; for example, crystal grains (nanocrystals)may be included in the amorphous structure. Even when the oxidesemiconductor layer is changed into a microcrystalline film or apolycrystalline film, the thin film transistor can obtain switchingcharacteristics as long as the oxide semiconductor layer is placed intoa state where oxygen is in excess to have a higher resistance, i.e., tobecome an i-type.

However, the oxide semiconductor layer is preferably amorphous in orderto reduce off-current of the TFT and to achieve low power consumption.

In order to be amorphous even after the heat treatment for dehydrationor dehydrogenation which follows the formation of the oxidesemiconductor layer, the oxide semiconductor layer preferably has asmall thickness of less than or equal to 50 nm. By making the thicknessof the oxide semiconductor layer small, crystallization in the oxidesemiconductor layer at the time of the heat treatment after theformation thereof can be suppressed.

Alternatively, in order to be amorphous even after the heat treatmentfor dehydration or dehydrogenation which follows the formation of theoxide semiconductor layer, the oxide semiconductor layer is made toinclude silicon oxide (SiO_(x) (X>0)) which inhibits crystallization,and thus can be prevented from being crystallized when the heattreatment is performed after the oxide semiconductor layer is formed inthe manufacturing process.

Note that in this specification, off current is current which flowsbetween a source electrode and a drain electrode when a transistor is inan off state. For example, in an n-channel transistor, the off currentis current which flows between a source electrode and a drain electrodewhen gate voltage is lower than threshold voltage of the transistor.

Further, a gate electrode layer, a source electrode layer, a drainelectrode layer, a pixel electrode layer, another electrode layer, oranother wiring layer can be formed by a sputtering method, a vacuumevaporation method (such as an electron beam evaporation method), an arcdischarge ion plating method or a spray method using a conductivematerial having a visible light transmitting property such as thefollowing metal oxide: an In—Sn—Zn—O-based metal oxide; anIn—Al—Zn—O-based metal oxide; a Sn—Ga—Zn—O-based metal oxide; anAl—Ga—Zn—O-based metal oxide; a Sn—Al—Zn—O-based metal oxide; anIn—Zn—O-based metal oxide; a Sn—Zn—O-based metal oxide; an Al—Zn—O-basedmetal oxide; an In—O-based metal oxide; a Sn—O-based metal oxide; and aZn—O-based metal oxide. Further, the silicon oxide may be included in awiring layer or an electrode layer which is formed of the above metaloxide.

As other materials which may be used for the gate electrode layer, thesource electrode layer, the drain electrode layer, the pixel electrodelayer, another electrode layer, or another wiring layer, anAl—Zn—O-based non-single-crystal film including nitrogen, which is anAl—Zn—O—N-based non-single-crystal film, a Zn—O—N-basednon-single-crystal film including nitrogen, or a Sn—Zn—O—N-basednon-single-crystal film including nitrogen may be used. Note that therelative proportion (atomic %) of zinc in an Al—Zn—O—N-based oxidesemiconductor film is less than or equal to 47 atomic % and is largerthan the relative proportion (atomic %) of aluminum in the oxidesemiconductor film. The relative proportion (atomic %) of aluminum inthe oxide semiconductor film is larger than the relative proportion(atomic %) of nitrogen in a conductive film having a light transmittingproperty. Note that the unit of the relative proportion in theconductive film having a light transmitting property is atomic percent,and the relative proportion is evaluated by analysis using an electronprobe X-ray microanalyzer (EPMA).

An aperture ratio of a display device can be improved by using aconductive film having a visible light transmitting property for thegate electrode layer, the source electrode layer, the drain electrodelayer, the pixel electrode layer, another electrode layer, or anotherwiring layer. In addition, when a material having a light transmittingproperty is used also for the oxide semiconductor layer, the apertureratio can be further improved. By using a film having a lighttransmitting property for components (a wiring and a semiconductorlayer) of a thin film transistor, particularly in a small liquid crystaldisplay device, a high aperture ratio can be achieved even when the sizeof a pixel is miniaturized for an increase in the number of scan lines,for example, so as to realize high definition of a display image.Further, by using a film having a light transmitting property forcomponents of a thin film transistor, a high aperture ratio can beachieved even when one pixel is divided into a plurality of sub-pixelsin order to realize a wide viewing angle. In other words, an apertureratio can be high even when a group of thin film transistors is denselyarranged and an area of a display region can be sufficiently secured.For example, in the case where one pixel includes two to foursub-pixels, an aperture ratio can be improved because not only the thinfilm transistor but also their respective storage capacitor has a lighttransmitting property.

Also in a light-emitting display device, a high aperture ratio can beachieved by using a film having a light transmitting property forcomponents (a wiring and a semiconductor layer) of a thin filmtransistor even when a plurality of thin film transistors is placed inone pixel. In a light-emitting display device using a light-emittingelement, a plurality of thin film transistors is included in a pixelportion, and a portion in which a gate electrode of a thin filmtransistor is electrically connected to a source wiring or a drainwiring of another transistor is also included in the pixel portion. Forexample, even when two to seven thin film transistors and a storagecapacitor are included in one pixel in a light-emitting display device,a high aperture ratio can be achieved because the thin film transistorsand the storage capacitor have a light transmitting property.

In addition, when the gate electrode layer, the source electrode layer,the drain electrode layer, the pixel electrode layer, another electrodelayer, and another wiring layer are formed using the same material, acommon sputtering target and a common manufacturing apparatus can beused; therefore, cost of the material of these layers and an etchant (oran etching gas) which is used in etching can be reduced, resulting in areduction in manufacturing cost.

In this specification, a film having a visible light transmittingproperty refers to a film with a thickness which realizes a visiblelight transmittance of 75% to 100%. Such a film is referred to also as atransparent conductive film. A conductive film which is semitransparentto visible light may be used as metal oxide for the gate electrodelayer, the source electrode layer, the drain electrode layer, the pixelelectrode layer, another electrode layer, or another wiring layer. Whena conductive film is semitransparent to visible light, it has atransmittance of visible light of 50% to 75%.

The thickness of each of the gate electrode layer, the source electrodelayer, the drain electrode layer, the pixel electrode layer, anotherelectrode layer, and another wiring layer is set to greater than orequal to 30 nm and less than or equal to 200 nm. A thickness whichallows each layer to have a light transmitting property or to besemitransparent to visible light may be selected.

Further, the gate insulating layer and the oxide semiconductor film maybe processed successively (also referred to as successive processing, insitu process, or successive formation) without exposure to air. When thegate insulating layer and the oxide semiconductor film are successivelyprocessed without exposure to air, the gate insulating layer and theoxide semiconductor film can be formed without contamination of aninterface thereof by atmospheric components or impurity elementsfloating in air, such as moisture or hydrocarbon. Therefore, variationin characteristics between the thin film transistors can be reduced.

Note that the term “successive processing” in this specification meansthat during a series of a first treatment step by a PCVD method or asputtering method to a second treatment step by a PCVD method or asputtering method, an atmosphere in which a substrate to be processed isdisposed is not contaminated by a contaminant atmosphere such as air,and is constantly controlled to be vacuum, an inert gas atmosphere (anitrogen atmosphere or a rare gas atmosphere) or an oxygen atmosphere.By the successive processing, film formation or the like can beperformed while preventing moisture or the like from attaching again tothe substrate to be processed which is cleaned.

Performing the process from the first treatment step to the secondtreatment step in the same chamber is within the scope of the successiveprocessing in this specification.

In addition, the following is also within the scope of the successiveprocessing in this specification: in the case of performing the processfrom the first treatment step to the second treatment step in differentchambers, the substrate is transferred after the first treatment step toanother chamber without being exposed to air and subjected to the secondtreatment.

Note that between the first treatment step and the second treatmentstep, a substrate transfer step, an alignment step, a slow-cooling step,a step of heating or cooling the substrate to a temperature which isnecessary for the second step or the like may be provided. Such aprocess is also within the scope of the successive processing in thisspecification.

A step in which liquid is used, such as a cleaning step, wet etching, orresist formation, may be provided between the first treatment step andthe second treatment step. This case is not within the scope of thesuccessive treatment in this specification.

A semiconductor device having a structure which is obtained by the abovemanufacturing method is a semiconductor device including: a gateelectrode layer over a substrate having an insulating surface; a gateinsulating layer over the gate electrode layer; an oxide semiconductorlayer over the gate insulating layer; a source electrode layer and adrain electrode layer over the oxide semiconductor layer; a protectiveinsulating layer in contact with part of the oxide semiconductor layer,over the gate insulating layer, the oxide semiconductor layer, thesource electrode layer, and the drain electrode layer; and a pixelelectrode layer including metal oxide over the protective insulatinglayer. In the above structure, the gate electrode layer, the gateinsulating layer, the oxide semiconductor layer, the source electrodelayer, the drain electrode layer, the protective insulating layer, andthe pixel electrode layer have a light transmitting property. The pixelelectrode layer overlaps with the oxide semiconductor layer and the gateelectrode layer.

With the above structure, at least one of the above problems isresolved.

In the above structure, although the pixel electrode layer overlaps withthe oxide semiconductor layer and the gate electrode layer, theoverlapping region can also serve as a display region, whereby a highaperture ratio can be realized. The pixel electrode layer which overlapswith the oxide semiconductor layer and the gate electrode layer may be apixel electrode layer of an adjacent pixel. In other words, a structurecan be employed in which the pixel electrode layer electricallyconnected to the oxide semiconductor layer with the source electrodelayer and the drain electrode layer therebetween overlaps not with thechannel formation region of the oxide semiconductor layer but with thepixel electrode layer of an adjacent pixel.

In a terminal portion in which a plurality of terminal electrodesconnected to an external terminal such as an FPC is placed, the terminalelectrodes can be formed using the same material and process as the gateelectrode layer, the source electrode layer, the drain electrode layer,or the pixel electrode layer.

Further, the storage capacitors in the pixel portion of the liquidcrystal display device or the light-emitting display device include acapacitor wiring layer which is formed of a conductive material having avisible light transmitting property, a capacitor electrode layer whichis formed of a conductive material having a visible light transmittingproperty, and the gate insulating layer that is used as a dielectric.Note that the capacitor wiring layer in this case can be formed usingthe same material and process as the gate electrode layer. In addition,the capacitor electrode layer can be formed using the same material andprocess as the source electrode layer or the drain electrode layer.

Alternatively, the storage capacitors in the pixel portion of the liquidcrystal display device or the light-emitting display device may includea capacitor wiring layer formed of a conductive material having avisible light transmitting property, a pixel electrode layer which isformed of a conductive material having a visible light transmittingproperty, and the protective insulating layer that is used as adielectric. The capacitor wiring layer in this case can be formed usingthe same material and process as the source electrode layer or the drainelectrode layer.

Moreover, as a display device including a thin film transistor, alight-emitting display device in which a light-emitting element is usedand a display device in which an electrophoretic display element isused, which is also referred to as electronic paper, are given inaddition to a liquid crystal display device.

There is no particular limitation on the foregoing liquid crystaldisplay device, and a liquid crystal display device using TN liquidcrystal, IPS liquid crystal, OCB liquid crystal, STN liquid crystal, VAliquid crystal, ECB liquid crystal, GH liquid crystal, polymer dispersedliquid crystal, discotic liquid crystal, or the like can be used. Amongthem, a normally black liquid crystal panel, such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modeis preferable. Some examples are given as a vertical alignment mode. Forexample, an MVA (multi-domain vertical alignment) mode, a PVA (patternedvertical alignment) mode, an ASV mode, or the like can be employed.Specifically, one pixel is divided into a plurality of sub-pixels and aprojection portion is provided in a position of a counter substratecorresponding to the center of each sub-pixel, so that a multi-domainpixel is formed. Such a driving method, in which one pixel is dividedinto a plurality of sub-pixels and a projection portion is provided at aposition of a counter substrate which corresponds to the center of eachsub-pixel so that orientation division (multi-domain) of one pixel isperformed and a wide viewing angle is achieved, is referred to assub-pixel driving. It is to be noted that the projection portion may beprovided over/on either one or both of the counter substrate and theelement substrate. The projection portion makes liquid crystal moleculesorient radially and improves controllability of the orientation.

Further, an electrode for driving liquid crystal, that is, a pixelelectrode may have a top view shape like a comb-shape or a zigzaggedshape so that a direction in which voltage is applied may be varied.Further, a multi-domain pixel may be formed utilizing photo-alignment.

Since a thin film transistor is easily broken due to static electricityor the like, a protective circuit for protecting the thin filmtransistor in the pixel portion is preferably provided over the samesubstrate for a gate line or a source line. The protective circuit ispreferably formed with a non-linear element including an oxidesemiconductor.

In a pixel portion of a display device, by using a film having a lighttransmitting property for components of a thin film transistor, a highaperture ratio can be achieved even when the size of a pixel isminiaturized for an increase in the number of scan lines, for example,so as to realize high definition of a display image. Further, by using afilm having a light transmitting property for components of a thin filmtransistor, a high aperture ratio can be achieved even when one pixel isdivided into a plurality of sub-pixels in order to realize a wideviewing angle.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are cross-sectional views illustrating manufacturingsteps of one embodiment of the present invention;

FIGS. 2A and 2B are a plan view and a cross-sectional view,respectively, illustrating one embodiment of the present invention;

FIGS. 3A to 3D are cross-sectional views illustrating manufacturingsteps of one embodiment of the present invention;

FIGS. 4A to 4C are cross-sectional views illustrating manufacturingsteps of one embodiment of the present invention;

FIGS. 5A to 5C are cross-sectional views illustrating manufacturingsteps of one embodiment of the present invention;

FIGS. 6A and 6B are a plan view and a cross-sectional view,respectively, illustrating one embodiment of the present invention;

FIGS. 7A and 7B are cross-sectional views illustrating manufacturingsteps of one embodiment of the present invention, and FIG. 7C is a planview illustrating one embodiment of the present invention;

FIGS. 8A to 8D are cross-sectional views illustrating manufacturingsteps of one embodiment of the present invention;

FIGS. 9A to 9C are cross-sectional views illustrating manufacturingsteps of one embodiment of the present invention;

FIG. 10 is a plan view illustrating one embodiment of the presentinvention;

FIG. 11 is a plan view illustrating one embodiment of the presentinvention;

FIGS. 12A and 12C are cross-sectional views and FIGS. 12B and 12D areplan views illustrating one embodiment of the present invention;

FIGS. 13A to 13C are perspective views illustrating one embodiment ofthe present invention;

FIGS. 14A and 14B are block diagrams illustrating one embodiment of thepresent invention;

FIG. 15 is a timing chart illustrating one embodiment of the presentinvention;

FIG. 16 is an equivalent circuit diagram of a pixel in a semiconductordevice;

FIGS. 17A to 17C are cross-sectional views each illustrating asemiconductor device;

FIGS. 18A and 18B are diagrams each illustrating a semiconductor device;

FIGS. 19A and 19B are diagrams each illustrating a semiconductor device;and

FIG. 20 is a diagram illustrating a semiconductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. Therefore, the present inventionis not construed as being limited to description of the embodiments.

Embodiment 1

A semiconductor device and a method for manufacturing a semiconductordevice will be described with reference to FIGS. 1A to 1C and FIGS. 2Aand 2B. In FIG. 2B, a thin film transistor 470 which is one type of astructure called a channel-etch type is illustrated.

FIG. 2A is a plan view of the thin film transistor 470 included in asemiconductor device, and FIG. 2B is a cross-sectional view taken alongline C1-C2 of FIG. 2A. The thin film transistor 470 is a bottom gatethin film transistor and includes, over a substrate 400 having aninsulating surface, a gate electrode layer 401, a gate insulating layer402, an oxide semiconductor layer 403, a source electrode layer 405 a,and a drain electrode layer 405 b. In addition, an oxide insulating film407 is provided to cover the thin film transistor 470 and be in contactwith the oxide semiconductor layer 403.

The substrate 400 having an insulating surface, the gate electrode layer401, the gate insulating layer 402, the oxide semiconductor layer 403,the source electrode layer 405 a, the drain electrode layer 405 b, andthe oxide insulating film 407 are all formed using a material having avisible light transmitting property. Thus, the thin film transistor 470has a light transmitting property and an aperture ratio can be improvedin the case where the thin film transistor 470 is placed in a pixelportion of a display device.

As for the oxide semiconductor layer 403, at least after an oxidesemiconductor film is formed, a heat treatment (a heat treatment fordehydration or dehydrogenation) for reducing moisture and the like whichare impurities is performed. The heat treatment for dehydration ordehydrogenation and slow cooling follow the formation of an oxideinsulating film in contact with the oxide semiconductor layer, and thelike; accordingly, the carrier of the oxide semiconductor layer isreduced to lead to an improvement in the reliability of the thin filmtransistor 470.

Impurities such as moisture are reduced not only in the oxidesemiconductor layer 403 but also in the gate insulating layer 402, atinterfaces between the oxide semiconductor layer 403 and a film aboveand in contact therewith and between the oxide semiconductor layer 403and a film below and in contact therewith which are specifically aninterface between the gate insulating layer 402 and the oxidesemiconductor layer 403 and an interface between the oxide insulatingfilm 407 and the oxide semiconductor layer 403.

Hereinafter, cross-sectional views which describe the manufacturingprocess of the thin film transistor 470 illustrated in FIG. 2B are inFIGS. 1A to 1C.

In FIG. 1A, the gate electrode layer 401 is provided over the substrate400 having an insulating surface.

Although there is no particular limitation on a substrate which can beused as the substrate 400 having an insulating surface, it is necessarythat the substrate have at least enough heat resistance to a heattreatment to be performed later. As the substrate 400 having aninsulating surface, a glass substrate formed of barium borosilicateglass, aluminoborosilicate glass, or the like can be used.

In the case where a glass substrate is used and the temperature at whichthe heat treatment is to be performed later is high, a glass substratewhose strain point is greater than or equal to 730° C. is preferablyused. As a glass substrate, a glass material such as aluminosilicateglass, aluminoborosilicate glass, or barium borosilicate glass is used,for example. Note that by containing a larger amount of barium oxide(BaO) than boric acid, a glass substrate is heat-resistant and of morepractical use. Therefore, a glass substrate containing BaO and B₂O₃ sothat the amount of BaO is larger than that of B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the above glass substrate. Alternatively, crystallized glassor the like can be used.

Further, an insulating film serving as a base film may be providedbetween the substrate 400 and the gate electrode layer 401. The basefilm has a function of preventing diffusion of an impurity element fromthe substrate 400, and can be formed to have a single-layer orstacked-layer structure using one or more of a silicon nitride film, asilicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

As the material of the gate electrode layer 401, a conductive materialhaving a visible light transmitting property such as the followingmaterials can be employed: an In—Sn—Zn—O-based metal oxide; anIn—Al—Zn—O-based metal oxide; a Sn—Ga—Zn—O-based metal oxide; anAl—Ga—Zn—O-based metal oxide; a Sn—Al—Zn—O-based metal oxide; anIn—Zn—O-based metal oxide; a Sn—Zn—O-based metal oxide; an Al—Zn—O-basedmetal oxide; an In—O-based metal oxide; a Sn—O-based metal oxide; and aZn—O-based metal oxide. The thickness of the gate electrode layer 401 isselected as appropriate to be within the range of 30 nm to 200 nm. As adeposition method of the metal oxide used for the gate electrode layer401, a sputtering method, a vacuum evaporation method (an electron beamevaporation method or the like), an arc discharge ion plating method ora spray method is used.

Next, the gate insulating layer 402 is formed over the gate electrodelayer 401.

The gate insulating layer 402 can be formed to have a single-layer orstacked layer structure using a silicon oxide layer, a silicon nitridelayer, a silicon oxynitride layer, or a silicon nitride oxide layer by aplasma CVD method, a sputtering method, or the like. For example, asilicon oxynitride layer may be formed using SiH₄, oxygen, and nitrogenas a film formation gas by a plasma CVD method.

Then, an oxide semiconductor film is formed to a thickness of greaterthan or equal to 2 nm and less than or equal to 200 nm over the gateinsulating layer 402.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust on a surface of the gate insulating layer 402 is preferablyremoved by reverse sputtering in which an argon gas is introduced andplasma is generated. The reverse sputtering refers to a method in which,without application of a voltage to a target side, an RF power source isused for application of a voltage to a substrate side in an argonatmosphere to generate plasma in the vicinity of the substrate to modifya surface. Note that instead of argon, nitrogen, helium, oxygen or thelike may be used.

The oxide semiconductor film is formed by a sputtering method with useof an In—Ga—Zn—O-based oxide semiconductor target. Alternatively, theoxide semiconductor film can be formed by a sputtering method in a raregas (typically argon) atmosphere, in an oxygen atmosphere, or in anatmosphere including a rare gas (typically argon) and oxygen.

The gate insulating layer 402 and the oxide semiconductor film may beformed successively without exposure to air. When the gate insulatinglayer 402 and the oxide semiconductor film are successively formedwithout exposure to air, the gate insulating layer 402 and the oxidesemiconductor film can be formed without contamination of an interfacethereof by atmospheric components or impurity elements floating in air,such as moisture or hydrocarbon. Therefore, variation in characteristicsbetween the thin film transistors can be reduced.

Then, the oxide semiconductor film is processed into an oxidesemiconductor layer (a first oxide semiconductor layer 430), which is anisland-shaped oxide semiconductor layer, by a photolithography step (seeFIG. 1A).

Next, the first oxide semiconductor layer 430 is dehydrated ordehydrogenated. A temperature at which a first heat treatment fordehydration or dehydrogenation is performed is greater than or equal to350° C. and less than the strain point of the substrate, preferablygreater than or equal to 400° C. Here, the substrate is introduced intoan electric furnace which is one of heat treatment apparatuses and thefirst oxide semiconductor layer 430 is subjected to a heat treatment inan oxygen atmosphere; then, the slow cooling is performed in the oxygenatmosphere, whereby a second oxide semiconductor layer 431 is formed(see FIG. 1B). The slow cooling is performed from the heatingtemperature T at which the oxide semiconductor layer is dehydrated ordehydrogenated to a temperature low enough to prevent water from comingin again, specifically to a temperature which is more than 100° C. lowerthan the heating temperature T. Alternatively, the slow cooling isperformed to a temperature lower than a temperature of a second heattreatment which is to be performed later and then the substrate is takenout of the heat treatment apparatus. The oxide semiconductor layer issubjected to the heat treatment in the oxygen atmosphere, whereby animpurity such as water contained in the oxide semiconductor layer can beremoved and at the same time, the second oxide semiconductor layer 431is placed into a state where oxygen is in excess. The oxidesemiconductor layer is crystallized and changed into a microcrystallinefilm or a polycrystalline film in some cases depending on the conditionof the first heat treatment or the material of the oxide semiconductorlayer.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the oxygen gas.Alternatively, the purity of the oxygen gas which is introduced into theheat treatment apparatus is preferably greater than or equal to 6N(99.9999%) or more preferably greater than or equal to 7N (99.99999%)(that is, the impurity concentration in the oxygen is less than or equalto 1 ppm, or preferably less than or equal to 0.1 ppm).

The first heat treatment is performed for greater than or equal to 0.5hours and less than or equal to 10 hours, where the rate of temperatureincrease of the electric furnace is preferably greater than or equal to0.1° C./min and less than or equal to 20° C./min. Further, the rate oftemperature decrease in the electric furnace is preferably greater thanor equal to 0.1° C./min and less than or equal to 15° C./min.

As a result, the reliability of the thin film transistor to be formedlater can be improved.

Further, instead of the heating method in which an electric furnace isused, a rapid heating method such as a gas rapid thermal anneal (GRTA)method using a heated gas or a lamp rapid thermal anneal (LRTA) methodusing lamp light can be used for the first heat treatment.

In the case where the heat treatment apparatus is a multi-chamber type,a chamber for the first heat treatment can be different from that for acooling treatment. Typically, the oxide semiconductor layer over thesubstrate is heated in a first chamber which is filled with an oxygengas and whose temperature is increased to greater than or equal to 400°C. and less than the strain point of the substrate. Then, through atransfer chamber into which an oxygen gas is introduced, the substrateon which the above first heat treatment is performed is transferred intoa second chamber which is filled with oxygen and whose temperature isless than or equal to 100° C. or preferably room temperature, and issubjected to the cooling treatment. Through the above steps, throughputcan be improved.

Alternatively, the oxide semiconductor film before being processed intothe island-shaped oxide semiconductor layer can also be subjected to thefirst heat treatment in an oxygen atmosphere. In that case, after thefirst heat treatment and the cooling treatment of the oxidesemiconductor film, the substrate is taken out of the heating device anda photolithography step is performed.

Before the oxide semiconductor film is formed, the gate insulating layermay be subjected to a heat treatment (at a temperature greater than orequal to 400° C. and less than the strain point of the substrate) in aninert gas (nitrogen or a rare gas such as helium, neon, or argon)atmosphere, in an oxygen atmosphere, or under a reduced pressure so thatan impurity such as hydrogen and water in the layer is removed.

Next, a conductive film is formed over the gate insulating layer 402 andthe second oxide semiconductor layer 431. As a deposition method of theconductive film, a sputtering method, a vacuum evaporation method (anelectron beam evaporation method or the like), an arc discharge ionplating method, or a spray method is used.

As the material of the conductive film, a conductive material having avisible light transmitting property such as the following materials canbe employed: an In—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metaloxide; a Sn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide;a Sn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; aSn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-basedmetal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide. Thethickness of the conductive film is selected as appropriate to be withinthe range of 30 nm to 200 nm.

Then, the second oxide semiconductor layer 431 and the conductive filmare selectively etched by a photolithography step so as to form theoxide semiconductor layer 403, the source electrode layer 405 a, and thedrain electrode layer 405 b. Note that only part of the oxidesemiconductor layer is etched to be the oxide semiconductor layer havinga groove (depression). When oxygen ashing is performed at the time ofremoving a resist mask which is used at this photolithography step,oxygen is introduced into an exposed region of the oxide semiconductorlayer.

Next, the oxide insulating film 407 serving as a protective insulatinglayer is formed in contact with part of the oxide semiconductor layer403, whereby the thin film transistor 470 can be manufactured (see FIG.1C). The oxide insulating film 407 is formed to have a thickness of atleast greater than or equal to 1 nm and can be appropriately formed by amethod in which an impurity such as water or hydrogen is prevented fromentering the oxide insulating film 407, for example, by a CVD method ora sputtering method. Here, the oxide insulating film 407 is formed by asputtering method. The oxide insulating film 407, which is formed incontact with the low-resistance oxide semiconductor layer, does notcontain an impurity such as moisture, a hydrogen ion, or OH⁻ and isformed using an inorganic insulating film which prevents intrusion ofthese from the outside. Typically, a silicon oxide film, a siliconnitride oxide film, an aluminum oxide film, or an aluminum oxynitridefilm is used. Alternatively, a silicon nitride film or an aluminumnitride film on and in contact with the oxide insulating film 407 may beformed. The silicon nitride film does not contain an impurity such asmoisture, a hydrogen ion, or OH⁻ and prevents intrusion of these fromthe outside.

In this embodiment, a silicon oxide film with a thickness of 300 nm isformed as the oxide insulating film 407. A substrate temperature at thetime of the film formation may be greater than or equal to roomtemperature and less than or equal to 300° C.; in this embodiment, thesubstrate temperature is 100° C. The silicon oxide film can be formed bya sputtering method in a rare gas (typically argon) atmosphere, in anoxygen atmosphere, or in an atmosphere including a rare gas (typicallyargon) and oxygen. As a target, a silicon oxide target or a silicontarget can be used; for example, the silicon oxide film can be formedusing a silicon target by a sputtering method in an atmosphere includingoxygen and nitrogen.

Further, after the oxide insulating film 407 is formed, the thin filmtransistor 470 may be subjected to the second heat treatment (preferablyat a temperature greater than or equal to 150° C. and less than 350° C.)in a nitrogen atmosphere or in an oxygen atmosphere. For example, thesecond heat treatment is performed in a nitrogen atmosphere at 250° C.for one hour. By the second heat treatment, the oxide semiconductorlayer 403 is heated while being in contact with the oxide insulatingfilm 407; thus, variation in electric characteristics of the thin filmtransistor 470 can be reduced.

Embodiment 2

A semiconductor device and a method for manufacturing a semiconductordevice which are different from Embodiment 1 will be described withreference to FIGS. 3A to 3D. The portion that is identical to or has afunction similar to those described in Embodiment 1 can be formed in amanner similar to that described in Embodiment 1; therefore, repetitivedescription is omitted.

FIGS. 3A to 3D are cross-sectional views illustrating a manufacturingprocess of a thin film transistor 480. The structure of the thin filmtransistor 480 which is illustrated in FIG. 3D is called an invertedcoplanar type (also called a bottom-contact type).

Similarly to Embodiment 1, the gate electrode layer 401 is provided overthe substrate 400 having an insulating surface. An insulating filmserving as a base film may be provided between the substrate 400 and thegate electrode layer 401.

Next, similarly to Embodiment 1, the gate insulating layer 402 is formedover the gate electrode layer 401. Then, an oxide semiconductor film isformed over the gate insulating layer 402.

The oxide semiconductor film is then processed into an oxidesemiconductor layer (the first oxide semiconductor layer 430), which isan island-shaped oxide semiconductor layer, by a photolithography step(see FIG. 3A). Note that FIG. 3A is the same as FIG. 1A.

Next, the first oxide semiconductor layer 430 is dehydrated ordehydrogenated. A temperature at which a first heat treatment fordehydration or dehydrogenation is performed is greater than or equal to350° C. and less than the strain point of the substrate, preferablygreater than or equal to 400° C. Here, the substrate is introduced intoan electric furnace which is one of heat treatment apparatuses and thefirst oxide semiconductor layer 430 is subjected to a heat treatment inan inert gas (nitrogen or a rare gas such as helium, neon, or argon)atmosphere or under a reduced pressure, whereby a second oxidesemiconductor layer 442 is formed (see FIG. 3B). By the heat treatmentin an inert gas atmosphere or under a reduced pressure, the resistanceof the oxide semiconductor layer is reduced (a carrier concentrationthereof is increased preferably to greater than or equal to 1×10¹⁸/cm³)and a low-resistance oxide semiconductor layer (the second oxidesemiconductor layer 442) can be formed.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the nitrogen or the rare gassuch as helium, neon, or argon. Alternatively, the purity of thenitrogen or the rare gas such as helium, neon, or argon which isintroduced into the heat treatment apparatus is preferably greater thanor equal to 6N, or more preferably greater than or equal to 7N (that is,the impurity concentration is less than or equal to 1 ppm, or preferablyless than or equal to 0.1 ppm). In this embodiment, the dehydration ordehydrogenation is performed by heating the electric furnace, which hasa nitrogen atmosphere and to which the substrate is introduced, to atemperature greater than or equal to 350° C. and less than or equal to600° C., or preferably greater than or equal to 400° C.; then,introduction of the nitrogen or the rare gas is stopped and a heater isturned off.

After the heating, slow cooling is performed in an oxygen atmosphere sothat a third oxide semiconductor layer 481 is formed (see FIG. 3C). Theslow cooling is performed in an oxygen atmosphere from the heatingtemperature T at which the oxide semiconductor layer is dehydrated ordehydrogenated to a temperature low enough to prevent water from comingin again, specifically to a temperature more than 100° C. lower than theheating temperature T. Alternatively, the slow cooling is performed inan oxygen atmosphere to a temperature lower than a temperature of asecond heat treatment which is to be performed later and then thesubstrate is taken out of the heat treatment apparatus. In thisembodiment, the slow cooling is performed after the heater of theelectric furnace is turned off and oxygen is introduced into theelectric furnace. It is preferable that an impurity such as water orhydrogen be not contained in the introduced oxygen. Alternatively, thepurity of the oxygen which is introduced into a chamber from a gassupply source is less than or equal to 6N, or preferably less than orequal to 7N (that is, the impurity concentration in the oxygen is lessthan or equal to 1 ppm, or preferably less than or equal to 0.1 ppm).

As a result, the reliability of the thin film transistor to be formedlater can be improved.

Note that in the case where the first heat treatment is performed undera reduced pressure, cooling may be performed by introducing oxygen intothe electric furnace after the heat treatment and returning the pressureto an atmospheric pressure.

In the case where the heat treatment apparatus is a multi-chamber type,a chamber for the first heat treatment can be different from that for acooling treatment. Typically, the oxide semiconductor layer over thesubstrate is heated in a first chamber which is filled with nitrogen ora rare gas and whose temperature is increased to greater than or equalto 400° C. and less than the strain point of the substrate. Then, theslow cooling is performed to a temperature low enough to prevent waterfrom coming in again, specifically to a temperature which is more than100° C. lower than the heating temperature T. Next, through a transferchamber into which nitrogen or a rare gas is introduced, the substrateon which the above first heat treatment is performed is transferred intoa second chamber which is filled with oxygen and whose temperature isless than or equal to 100° C. or preferably room temperature, and issubjected to the cooling treatment. Through the above steps, throughputcan be improved.

Alternatively, the oxide semiconductor film before being processed intothe island-shaped oxide semiconductor layer can also be subjected to thefirst heat treatment in an inert gas atmosphere or under a reducedpressure. In that case, after the first heat treatment and the coolingtreatment, the substrate is taken out of the heating device and aphotolithography step is performed.

Before the oxide semiconductor film is formed, the gate insulating layermay be subjected to a heat treatment (at a temperature greater than orequal to 400° C. and less than the strain point of the substrate) in aninert gas (nitrogen or a rare gas such as helium, neon, or argon)atmosphere, in an oxygen atmosphere, or under a reduced pressure so thatan impurity such as hydrogen and water in the layer is removed.

Then, a conductive film is formed over the gate insulating layer 402 andthe third oxide semiconductor layer 481.

As the material of the conductive film, a conductive material having avisible light transmitting property such as the following materials canbe employed: an In—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metaloxide; a Sn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide;a Sn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; aSn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-basedmetal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide. Thethickness of the conductive film is selected as appropriate to be withinthe range of 30 nm to 200 nm.

Then, the third oxide semiconductor layer 481 and the conductive filmare selectively etched by a photolithography step so as to form an oxidesemiconductor layer 483, the source electrode layer 405 a and the drainelectrode layer 405 b. Note that only part of the oxide semiconductorlayer is etched to be the oxide semiconductor layer having a groove(depression). When oxygen ashing is performed at the time of removing aresist mask which is used at this photolithography step, oxygen isintroduced into an exposed region of the oxide semiconductor layer.

Next, the oxide insulating film 407 serving as a protective insulatinglayer is formed in contact with part of the oxide semiconductor layer483, whereby the thin film transistor 480 can be manufactured (see FIG.3D). The oxide insulating film 407 is formed to have a thickness of atleast greater than or equal to 1 nm and can be appropriately formed by amethod in which an impurity such as water or hydrogen is prevented fromentering the oxide insulating film 407, for example, by a CVD method ora sputtering method. Here, the oxide insulating film 407 is formed by asputtering method. The oxide insulating film 407, which is formed incontact with the low-resistance oxide semiconductor layer, does notcontain an impurity such as moisture, a hydrogen ion, or OH⁻ and isformed using an inorganic insulating film which prevents intrusion ofthese from the outside. Typically, a silicon oxide film, a siliconnitride oxide film, an aluminum oxide film, or an aluminum oxynitridefilm is used. Alternatively, a silicon nitride film or an aluminumnitride film on and in contact with the oxide insulating film 407 may beformed. The silicon nitride film does not contain an impurity such asmoisture, a hydrogen ion, or OH⁻ and prevents intrusion of these fromthe outside.

In this embodiment, a silicon oxide film with a thickness of 300 nm isformed as the oxide insulating film 407. A substrate temperature at thetime of the film formation may be greater than or equal to roomtemperature and less than or equal to 300° C.; in this embodiment, thesubstrate temperature is 100° C. The silicon oxide film can be formed bya sputtering method in a rare gas (typically argon) atmosphere, in anoxygen atmosphere, or in an atmosphere including a rare gas (typicallyargon) and oxygen. As a target, a silicon oxide target or a silicontarget can be used; for example, the silicon oxide film can be formedusing a silicon target by a sputtering method in an atmosphere includingoxygen and nitrogen.

Further, after the oxide insulating film 407 is formed, the thin filmtransistor 480 may be subjected to the second heat treatment (preferablyat a temperature greater than or equal to 150° C. and less than 350° C.)in a nitrogen atmosphere or in an oxygen atmosphere. For example, thesecond heat treatment is performed in a nitrogen atmosphere at 250° C.for one hour. By the second heat treatment, the oxide semiconductorlayer 483 is heated while being in contact with the oxide insulatingfilm 407; thus, variation in electric characteristics of the thin filmtransistor 480 can be reduced.

This embodiment can be freely combined with Embodiment 1.

Embodiment 3

A semiconductor device and a method for manufacturing a semiconductordevice which are different from Embodiments 1 and 2 will be describedwith reference to FIGS. 4A to 4C. The portion that is identical to orhas a function similar to those described in Embodiments 1 and 2 can beformed in a manner similar to that described in Embodiments 1 and 2;therefore, repetitive description is omitted.

FIGS. 4A to 4C are cross-sectional views illustrating a manufacturingprocess of a thin film transistor 440. The structure of the thin filmtransistor 440 which is illustrated in FIG. 4C is called a channel stoptype.

Similarly to Embodiment 1, the gate electrode layer 401 is provided overthe substrate 400 having an insulating surface. An insulating filmserving as a base film may be provided between the substrate 400 and thegate electrode layer 401.

Next, similarly to Embodiment 1, the gate insulating layer 402 is formedover the gate electrode layer 401. Then, an oxide semiconductor film isformed over the gate insulating layer 402.

Then, the oxide semiconductor film is processed into an oxidesemiconductor layer (the first oxide semiconductor layer 430), which isan island-shaped oxide semiconductor layer, by a photolithography step(see FIG. 4A). Note that FIG. 4A is the same as FIG. 1A.

Next, the first oxide semiconductor layer 430 is dehydrated ordehydrogenated. A temperature at which a first heat treatment fordehydration or dehydrogenation is performed is greater than or equal to350° C. and less than the strain point of the substrate, preferablygreater than or equal to 400° C. Here, the substrate is introduced intoan electric furnace which is one of heat treatment apparatuses and thefirst oxide semiconductor layer 430 is subjected to a heat treatment inan inert gas (nitrogen or a rare gas such as helium, neon, or argon)atmosphere or under a reduced pressure, whereby a second oxidesemiconductor layer 444 is formed (see FIG. 4B).

Note that in the first heat treatment, it is preferable that water,hydrogen and the like be not contained in the nitrogen or the rare gassuch as helium, neon, or argon. Alternatively, the purity of thenitrogen or the rare gas such as helium, neon, or argon which isintroduced into the heat treatment apparatus is preferably greater thanor equal to 6N (99.9999%) or more preferably greater than or equal to 7N(99.99999%) (that is, the impurity concentration is less than or equalto 1 ppm, or preferably less than or equal to 0.1 ppm). In thisembodiment, the dehydration or dehydrogenation is performed by heatingthe electric furnace, which has a nitrogen atmosphere and to which thesubstrate is introduced, to a temperature greater than or equal to 350°C. and less than or equal to 600° C., or preferably greater than orequal to 400° C.; then, a heater is turned off and slow cooling isperformed. By the heat treatment and the slow cooling in an inert gasatmosphere or under a reduced pressure, the resistance of the oxidesemiconductor layer is reduced (a carrier concentration thereof isincreased preferably to greater than or equal to 1×10¹⁸/cm³) and alow-resistance oxide semiconductor layer (the second oxide semiconductorlayer 444) can be formed.

Note that in the case where the heat treatment is performed under areduced pressure, cooling may be performed by introducing an inert gasinto the electric furnace after the heat treatment and returning thepressure to an atmospheric pressure.

In the case where the heat treatment apparatus is a multi-chamber type,a chamber for the heat treatment can be different from that for acooling treatment. Typically, the oxide semiconductor layer over thesubstrate is heated in a first chamber which is filled with nitrogen ora rare gas and whose temperature is increased to greater than or equalto 200° C. and less than or equal to 600° C., or preferably greater thanor equal to 400° C. and less than or equal to 450° C. Then, the slowcooling is performed to a temperature low enough to prevent water fromcoming in again, specifically to a temperature more than 100° C. lowerthan the heating temperature T. Next, through a transfer chamber intowhich nitrogen or a rare gas is introduced, the substrate on which theabove heat treatment is performed is transferred into a second chamberwhich is filled with nitrogen or a rare gas and whose temperature isless than or equal to 100° C. or preferably room temperature, and issubjected to the cooling treatment. Through the above steps, throughputcan be improved.

Alternatively, the oxide semiconductor film before being processed intothe island-shaped oxide semiconductor layer can also be subjected to theheat treatment in an inert gas atmosphere or under a reduced pressure.In that case, after the oxide semiconductor film is subjected to theheat treatment in the inert gas atmosphere or under a reduced pressure,the slow cooling to a temperature greater than or equal to roomtemperature and less than 100° C. is performed; then, the substrate istaken out of the heating device and a photolithography step isperformed.

Before the oxide semiconductor film is formed, the gate insulating layermay be subjected to a heat treatment (at a temperature greater than orequal to 400° C. and less than the strain point of the substrate) in aninert gas (nitrogen or a rare gas such as helium, neon, or argon)atmosphere, in an oxygen atmosphere, or under a reduced pressure so thatan impurity such as hydrogen and water in the layer is removed.

Then, a conductive film is formed over the gate insulating layer 402 andthe second oxide semiconductor layer 444.

As the material of the conductive film, a conductive material having avisible light transmitting property such as the following materials canbe employed: an In—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metaloxide; a Sn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide;a Sn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; aSn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-basedmetal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide. Thethickness of the conductive film is selected as appropriate to be withinthe range of 30 nm to 200 nm.

Then, the second oxide semiconductor layer 444 and the conductive filmare selectively etched by a photolithography step so as to form theoxide semiconductor layer, the source electrode layer 405 a, and thedrain electrode layer 405 b. Note that only part of the oxidesemiconductor layer is etched to be the oxide semiconductor layer havinga groove (depression). When oxygen ashing is performed at the time ofremoving a resist mask which is used at this photolithography step,oxygen is introduced into an exposed region of the oxide semiconductorlayer.

Next, the oxide insulating film 407 serving as a protective insulatinglayer is formed in contact with part of the oxide semiconductor layer.The oxide insulating film 407 is formed to have a thickness of at leastgreater than or equal to 1 nm and can be appropriately formed by amethod in which an impurity such as water or hydrogen is prevented fromentering the oxide insulating film 407, for example, by a CVD method ora sputtering method. Here, the oxide insulating film 407 is formed by asputtering method. The oxide insulating film 407, which is formed incontact with the low-resistance oxide semiconductor layer, does notcontain an impurity such as moisture, a hydrogen ion, or OH⁻ and isformed using an inorganic insulating film which prevents intrusion ofthese from the outside. Typically, a silicon oxide film, a siliconnitride oxide film, an aluminum oxide film, or an aluminum oxynitridefilm is used. Alternatively, a silicon nitride film or an aluminumnitride film on and in contact with the oxide insulating film 407 may beformed. The silicon nitride film does not contain an impurity such asmoisture, a hydrogen ion, or OH⁻ and prevents intrusion of these fromthe outside.

By forming the oxide insulating film 407 in contact with thelow-resistance second oxide semiconductor layer 444 by a sputteringmethod, a PCVD method, or the like, resistance is increased at least ina region of the low-resistance oxide semiconductor layer 444, which isin contact with the oxide insulating film 407 (a carrier concentrationthereof is decreased to preferably less than 1×10¹⁸/cm³), and thus theregion can be a high-resistance oxide semiconductor region. Further, theresistance of regions of the low-resistance oxide semiconductor layer444 which overlap with the source electrode layer 405 a and the drainelectrode layer 405 b, is still low, and thus two low-resistance oxidesemiconductor regions with the high-resistance oxide semiconductorregion therebetween are obtained. It is important to increase anddecrease the carrier concentration of the oxide semiconductor layer bythe heating in an inert gas atmosphere (or under a reduced pressure),the slow cooling, the formation of the oxide insulating film, and thelike during the manufacturing process of the semiconductor device. Theoxide semiconductor layer 444 becomes an oxide semiconductor layer 443(a third oxide semiconductor layer) which has the high-resistance oxidesemiconductor region and the low-resistance oxide semiconductor regions,and the thin film transistor 440 can be formed. Note that thehigh-resistance oxide semiconductor region serves as a channel formationregion of the thin film transistor 440.

Note that by forming the low-resistance oxide semiconductor regions inthe oxide semiconductor layer 443 which overlap with the drain andsource electrode layers, reliability can be increased when a drivecircuit is formed. Specifically, by forming the low-resistance oxidesemiconductor regions, a structure is realized in which the drainelectrode layer, the low-resistance oxide semiconductor region, and thechannel formation region can vary in conductivity in this order. Thus,in a transistor which operates while being connected to a wiring whichsupplies the drain electrode layer with a high power source potentialVDD, the low-resistance oxide semiconductor region serves as a buffer sothat a local high electric field is not applied even when a highelectric field is applied between the gate electrode layer and the drainelectrode layer; in this manner, the transistor can have a structurewith an increased withstand voltage.

In addition, by forming the low-resistance oxide semiconductor regionsin the oxide semiconductor layer 443 which overlap with the drain andsource electrode layers, leakage current in the channel formation regioncan be reduced when a drive circuit is formed. Specifically, by formingthe low-resistance oxide semiconductor regions, leakage current flowingbetween the drain electrode layer and the source electrode layer passesthrough the drain electrode layer, the low-resistance oxidesemiconductor region on the drain electrode layer side, the channelformation region, the low-resistance oxide semiconductor region on thesource electrode layer side, and the source electrode layer in thisorder. At this time, the leakage current which flows from thelow-resistance oxide semiconductor region on the drain electrode layerside to the channel formation region can be concentrated in the vicinityof the interface between the gate insulating layer and the channelformation region, which has high resistance when the transistor isturned off, whereby the leakage current at a back channel portion (partof the surface of the channel formation region which is apart from thegate electrode layer) can be reduced.

Further, after the oxide insulating film 407 is formed, the thin filmtransistor 440 may be subjected to a second heat treatment (preferablyat a temperature greater than or equal to 150° C. and less than 350° C.)in a nitrogen atmosphere or in an oxygen atmosphere. For example, thesecond heat treatment is performed in a nitrogen atmosphere at 250° C.for one hour. By the second heat treatment, the oxide semiconductorlayer 443 is heated while being in contact with the oxide insulatingfilm 407; thus, variation in electric characteristics of the thin filmtransistor 440 can be reduced.

This embodiment can be freely combined with Embodiment 1 or 2.

Embodiment 4

A semiconductor device and a method for manufacturing the semiconductordevice are described with reference to FIGS. 5A to 5C and FIGS. 6A and6B.

FIG. 6A is a plan view of a thin film transistor 460 included in asemiconductor device, and FIG. 6B is a cross-sectional view taken alongline D1-D2 of FIG. 6A. The thin film transistor 460 is a bottom gatethin film transistor and includes, over a substrate 450 having aninsulating surface, a gate electrode layer 451, a gate insulating layer452, a source electrode layer 455 a, a drain electrode layer 455 b, andan oxide semiconductor layer 453. In addition, an oxide insulating film457 is provided to cover the thin film transistor 460 and be in contactwith the oxide semiconductor layer 453. An In—Ga—Zn—O-basednon-single-crystal film is used for the oxide semiconductor layer 453.

In the thin film transistor 460, the gate insulating layer 452 exists inthe entire region including the thin film transistor 460, and the gateelectrode layer 451 is provided between the gate insulating layer 452and the substrate 450 which is a substrate having an insulating surface.The source electrode layer 455 a and the drain electrode layer 455 b areprovided over the gate insulating layer 452. Further, the oxidesemiconductor layer 453 is provided over the gate insulating layer 452,the source electrode layer 455 a and the drain electrode layer 455 b.Although not illustrated, a wiring layer is provided over the gateinsulating layer 452 in addition to the source electrode layer 455 a andthe drain electrode layer 455 b, and the wiring layer extends beyond theperipheral portion of the oxide semiconductor layer 453.

The substrate 450 having an insulating surface, the gate electrode layer451, the gate insulating layer 452, the oxide semiconductor layer 453,the source electrode layer 455 a, the drain electrode layer 455 b, andthe oxide insulating film 457 are all formed using a material having avisible light transmitting property. Thus, the thin film transistor 460has a light transmitting property and an aperture ratio can be improvedin the case where the thin film transistor 460 is placed in a pixelportion of a display device.

The oxide semiconductor layer 453 is subjected to a heat treatment (aheat treatment for dehydration or dehydrogenation) for reducing moistureand the like which are impurities and slow cooling at least after theoxide semiconductor film is formed; then, the oxide insulating film 457is formed in contact with the oxide semiconductor layer 453. In thismanner, the oxide semiconductor film is used as a channel formationregion.

FIGS. 5A to 5C are cross-sectional views illustrating steps ofmanufacturing the thin film transistor 460 which is illustrated in FIG.6B.

The gate electrode layer 451 is provided over the substrate 450 which isa substrate having an insulating surface. Further, an insulating filmserving as a base film may be provided between the substrate 450 and thegate electrode layer 451. The base film has a function of preventingdiffusion of an impurity element from the substrate 450, and can beformed to have a single-layer or stacked-layer structure using one ormore of a silicon nitride film, a silicon oxide film, a silicon nitrideoxide film, and a silicon oxynitride film.

As the material of the gate electrode layer 451, a conductive materialhaving a visible light transmitting property such as the followingmaterials can be employed: an In—Sn—Zn—O-based metal oxide; anIn—Al—Zn—O-based metal oxide; a Sn—Ga—Zn—O-based metal oxide; anAl—Ga—Zn—O-based metal oxide; a Sn—Al—Zn—O-based metal oxide; anIn—Zn—O-based metal oxide; a Sn—Zn—O-based metal oxide; an Al—Zn—O-basedmetal oxide; an In—O-based metal oxide; a Sn—O-based metal oxide; and aZn—O-based metal oxide. The thickness of the gate electrode layer 451 isselected as appropriate to be within the range of 30 nm to 200 nm.

Next, the gate insulating layer 452 is formed over the gate electrodelayer 451.

The gate insulating layer 452 can be formed to have a single-layer orstacked layer structure using a silicon oxide layer, a silicon nitridelayer, a silicon oxynitride layer, or a silicon nitride oxide layer by aplasma CVD method, a sputtering method, or the like. For example, asilicon oxynitride layer may be formed using SiH₄, oxygen, and nitrogenas a film formation gas by a plasma CVD method.

Then, a conductive film is formed over the gate insulating layer 452 andprocessed into the island-shaped source electrode layer 455 a and theisland-shaped drain electrode layer 455 b by a photolithography step(see FIG. 5A).

As the material of the conductive film, a conductive material having avisible light transmitting property such as the following materials canbe employed: an In—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metaloxide; a Sn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide;a Sn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; aSn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-basedmetal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide. Thethickness of the conductive film is selected as appropriate to be withinthe range of 30 nm to 200 nm.

Then, an oxide semiconductor film is formed over the gate insulatinglayer 452, the source electrode layer 455 a and the drain electrodelayer 455 b, and processed into an island-shaped oxide semiconductorlayer 483 (the first oxide semiconductor layer) by a photolithographystep (see FIG. 5B). When oxygen ashing is performed at the time ofremoving a resist mask which is used at this photolithography step,oxygen is introduced into an exposed region of the oxide semiconductorlayer.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust on a surface of the gate insulating layer 452 is preferablyremoved by reverse sputtering in which an argon gas is introduced andplasma is generated.

The oxide semiconductor layer 483 is subjected to the first heattreatment for dehydration or dehydrogenation. A temperature at which thefirst heat treatment for dehydration or dehydrogenation is performed isgreater than or equal to 350° C. and less than the strain point of thesubstrate, preferably greater than or equal to 400° C.

As the first heat treatment for dehydration or dehydrogenation, a heattreatment in an inert gas (nitrogen or a rare gas such as helium, neon,or argon) atmosphere, in an oxygen atmosphere, or under a reducedpressure is performed. After that, the slow cooling is performed fromthe heating temperature T at which the oxide semiconductor layer isdehydrated or dehydrogenated to a temperature low enough to preventwater from coming in again, specifically to a temperature which is morethan 100° C. lower than the heating temperature T. Alternatively, theslow cooling is performed to a temperature lower than a temperature of asecond heat treatment which is to be performed later and then thesubstrate is taken out of the heat treatment apparatus.

In this embodiment, similarly to Embodiment 1, the oxide semiconductorlayer 453 in which impurities such as moisture in the layer are reducedis formed by the first heat treatment performed in an oxygen atmosphereand the slow cooling performed in the oxygen atmosphere. There is noparticular limitation on the combination of the first heat treatment andthe slow cooling, and the combination and order described in any one ofEmbodiments 1 to 3 can be used.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the inert gas (nitrogen or arare gas such as helium, neon, or argon) atmosphere or in the oxygenatmosphere. Alternatively, the purity of the gas which is introducedinto the heat treatment apparatus is preferably greater than or equal to6N (99.9999%) or more preferably greater than or equal to 7N (99.99999%)(that is, the impurity concentration in the atmosphere is less than orequal to 1 ppm, or preferably less than or equal to 0.1 ppm).

As a result, the reliability of the thin film transistor to be formedlater can be improved.

Alternatively, the oxide semiconductor film before being processed intothe island-shaped oxide semiconductor layer can also be subjected to thefirst heat treatment in an oxygen atmosphere. In that case, after thefirst heat treatment and the cooling treatment of the oxidesemiconductor film, the substrate is taken out of the heating device anda photolithography step is performed.

Before the oxide semiconductor film is formed, the gate insulating layermay be subjected to a heat treatment (at a temperature greater than orequal to 400° C. and less than the strain point of the substrate) in aninert gas (nitrogen or a rare gas such as helium, neon, or argon)atmosphere, in an oxygen atmosphere, or under a reduced pressure so thatan impurity such as hydrogen and water in the layer is removed.

Next, the oxide insulating film 457 is formed in contact with the oxidesemiconductor layer 453 by a sputtering method or a PCVD method, wherebythe thin film transistor 460 can be manufactured (see FIG. 5C). In thisembodiment, a silicon oxide film with a thickness of 300 nm is formed asthe oxide insulating film 457. A substrate temperature at the time ofthe film formation may be greater than or equal to room temperature andless than or equal to 300° C.; in this embodiment, the substratetemperature is 100° C.

Further, after the oxide insulating film 457 is formed, the thin filmtransistor 460 may be subjected to the second heat treatment (preferablyat a temperature greater than or equal to 150° C. and less than 350° C.)in a nitrogen atmosphere or in an oxygen atmosphere. For example, thesecond heat treatment is performed at 250° C. for one hour in a nitrogenatmosphere. By the second heat treatment, the oxide semiconductor layer453 is heated while being in contact with the oxide insulating film 457;thus, variation in electric characteristics of the thin film transistor460 can be reduced.

This embodiment can be freely combined with Embodiment 1, 2, or 3.

Embodiment 5

In this embodiment, an example of a channel stop type thin filmtransistor 1430 is described with reference to FIGS. 7A to 7C. Anexample of a plan view of a thin film transistor is illustrated in FIG.7C, a cross-sectional view taken along dotted line Z1-Z2 of whichcorresponds to FIG. 7B. This embodiment is an example in which galliumis not included in an oxide semiconductor layer of the thin filmtransistor 1430.

First, a gate electrode layer 1401 is provided over a substrate 1400.

Additionally, an insulating film serving as a base film may be providedbetween the substrate 1400 and the gate electrode layer 1401. The basefilm has a function of preventing diffusion of an impurity element fromthe substrate 1400, and can be formed to have a single-layer orstacked-layer structure using one or more of a silicon nitride film, asilicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

As the material of the gate electrode layer 1401, a conductive materialhaving a visible light transmitting property such as the followingmaterials can be employed: an In—Sn—Zn—O-based metal oxide; anIn—Al—Zn—O-based metal oxide; a Sn—Ga—Zn—O-based metal oxide; anAl—Ga—Zn—O-based metal oxide; a Sn—Al—Zn—O-based metal oxide; anIn—Zn—O-based metal oxide; a Sn—Zn—O-based metal oxide; an Al—Zn—O-basedmetal oxide; an In—O-based metal oxide; a Sn—O-based metal oxide; and aZn—O-based metal oxide. The thickness of the gate electrode layer 1401is selected as appropriate to be within the range of 30 nm to 200 nm.

Next, a gate insulating layer 1402 is formed so as to cover the gateelectrode layer 1401. An oxide semiconductor layer is formed over thegate insulating layer 1402.

In this embodiment, the oxide semiconductor layer is formed using aSn—Zn—O-based oxide semiconductor by a sputtering method. When galliumis not used for the oxide semiconductor layer, use of an expensivetarget in formation of the oxide semiconductor layer can be avoided, sothat cost can be reduced.

Just after deposition of an oxide semiconductor film or after beingprocessed into an island-shaped oxide semiconductor layer, dehydrationor dehydrogenation is performed.

As the first heat treatment for dehydration or dehydrogenation, a heattreatment in an inert gas (nitrogen or a rare gas such as helium, neon,or argon) atmosphere, in an oxygen atmosphere, or under a reducedpressure is performed. A temperature at which the first heat treatmentis performed is greater than or equal to 350° C. and less than thestrain point of the substrate, preferably greater than or equal to 400°C. After that, slow cooling is performed from the heating temperature Tat which the oxide semiconductor layer is dehydrated or dehydrogenatedto a temperature low enough to prevent water from coming in again,specifically to a temperature which is more than 100° C. lower than theheating temperature T. Alternatively, the slow cooling is performed to atemperature lower than a temperature of a second heat treatment which isto be performed later and then the substrate is taken out of the heattreatment apparatus.

In this embodiment, similarly to Embodiment 1, an oxide semiconductorlayer 1403 in which impurities such as moisture in the layer are reducedis formed by the first heat treatment performed in an oxygen atmosphereand the slow cooling performed in the oxygen atmosphere (see FIG. 7A).There is no particular limitation on the combination of the first heattreatment and the slow cooling, and the combination and order describedin any one of Embodiments 1 to 3 can be used.

Note that in the first heat treatment, it is preferable that water,hydrogen and the like be not contained in the inert gas (nitrogen or arare gas such as helium, neon, or argon) atmosphere or in the oxygenatmosphere. Alternatively, the purity of the gas which is introducedinto the heat treatment apparatus is preferably greater than or equal to6N or more preferably greater than or equal to 7N (that is, the impurityconcentration in the atmosphere is less than or equal to 1 ppm, orpreferably less than or equal to 0.1 ppm).

As a result, the reliability of the thin film transistor to be formedlater can be improved.

Alternatively, the oxide semiconductor film before being processed intothe island-shaped oxide semiconductor layer can also be subjected to thefirst heat treatment in an oxygen atmosphere. In that case, after thefirst heat treatment and the cooling treatment of the oxidesemiconductor film, the substrate is taken out of the heating device anda photolithography step is performed.

Before the oxide semiconductor film is formed, the gate insulating layermay be subjected to a heat treatment (at a temperature greater than orequal to 400° C. and less than the strain point of the substrate) in aninert gas (nitrogen or a rare gas such as helium, neon, or argon)atmosphere, in an oxygen atmosphere, or under a reduced pressure so thatan impurity such as hydrogen and water in the layer is removed.

Next, a channel protective layer 1418 is provided on and in contact withthe oxide semiconductor layer 1403. By providing the channel protectivelayer 1418, damage to a channel formation region of the oxidesemiconductor layer 1403 (e.g., a reduction in thickness due to plasmaor an etchant in etching) can be prevented in the manufacturing process.Thus, the thin film transistor 1430 can have improved reliability.

Further, the channel protective layer 1418 can be successively formedwithout exposure to air after the heat treatment for dehydration ordehydrogenation. Successive film formation without exposure to air makesit possible to form the oxide semiconductor layer 1403 and the channelprotective layer 1418, the interface of which is not contaminated byatmospheric components or impurity elements floating in air, such asmoisture or hydrocarbon. Therefore, variation in characteristics betweenthe thin film transistors can be reduced.

The channel protective layer 1418 can be formed using an inorganicmaterial which contains oxygen (e.g., silicon oxide, silicon oxynitride,or silicon nitride oxide). As a method for forming the channelprotective layer 1418, a vapor deposition method such as a plasma CVDmethod or a thermal CVD method or a sputtering method can be used. Afterthe formation of the channel protective layer 1418, the shape thereof isprocessed by etching. Here, the channel protective layer 1418 is formedin such a manner that a silicon oxide film is formed by a sputteringmethod and processed by etching using a mask formed by photolithography.When oxygen ashing is performed at the time of removing a resist maskwhich is used at this photolithography step, oxygen is introduced intoan exposed region of the oxide semiconductor layer.

Then, a conductive film is formed over the channel protective layer 1418and the oxide semiconductor layer 1403.

As the material of the conductive film, a conductive material having avisible light transmitting property such as the following materials canbe employed: an In—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metaloxide; a Sn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide;a Sn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; aSn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-basedmetal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide. Thethickness of the conductive film is selected as appropriate to be withinthe range of 30 nm to 200 nm.

Next, the conductive film is selectively etched using a mask formed byphotolithography so as to form a source electrode layer 1405 a and adrain electrode layer 1405 b over the channel protective layer 1418 andthe oxide semiconductor layer 1403; thus, the thin film transistor 1430is manufactured (see FIG. 7B).

This embodiment can be freely combined with Embodiment 1, 2, or 3.

Embodiment 6

In this embodiment, a manufacturing example of a liquid crystal displaydevice in which the thin film transistor described in Embodiment 1 isplaced in a pixel portion is described with reference to FIGS. 8A to 8D,FIGS. 9A to 9C, FIG. 10, FIG. 11, FIGS. 12A to 12D, FIGS. 13A to 13C,FIGS. 14A and 14B, and FIG. 15.

In FIG. 8A, a glass substrate formed of barium borosilicate glass,aluminoborosilicate glass, or the like can be used as a substrate 100having a light transmitting property. As the substrate 100 having alight transmitting property, a large-area substrate having a size of,for example, 1000 mm×1200 mm, 1100 mm×1250 mm, or 1150 mm×1300 mm may beused. When such a large-area substrate is used, a plurality of liquidcrystal display devices can be manufactured using one substrate and themanufacturing cost can be reduced. In this embodiment, a glass substratehaving a size of 600 mm×720 mm is used.

Next, after a conductive film having a visible light transmittingproperty is formed over an entire surface of the substrate 100, a firstphotolithography step is performed. to form a resist mask, andunnecessary portions of the conductive film are removed by etching toform wirings and an electrode (a gate wiring including a gate electrodelayer 101, a capacitor wiring 108, and a first terminal 121). At thistime, the etching is performed so that at least end portions of the gateelectrode layer 101 have a tapered shape.

In the case of using a large-area substrate, instead of using anexpensive photomask for a photolithography, a resist mask may be formedby an ink-jet method. When the resist mask is formed by an ink-jetmethod, the manufacturing cost can be reduced. Note that a resist maskmay be formed by an ink-jet method in at least one step of thephotolithography process below in order to reduce the manufacturingcost.

As the material of the gate wiring including the gate electrode layer101, the capacitor wiring 108, and the first terminal 121 of a terminalportion, a conductive material having a visible light transmittingproperty such as the following materials can be employed: anIn—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metal oxide; aSn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide; aSn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; aSn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-basedmetal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide.Each thickness of the gate wiring including the gate electrode layer101, the capacitor wiring 108, and the first terminal 121 of a terminalportion is selected as appropriate to be within the range of 30 nm to200 nm. As a deposition method of the conductive film, a sputteringmethod, a vacuum evaporation method (an electron beam evaporation methodor the like), an arc discharge ion plating method, a spray method, or anink-jet method is used. In the case of forming the conductive film by anink-jet method, the photolithography step becomes unnecessary and afurther cost reduction can be achieved.

In this embodiment, as the conductive film, an In—Sn—O-based conductivefilm is formed using an In—Sn—O-based target by a sputtering method. Theconductive film may be subjected to a heat treatment in order to havelow resistance after being formed. The target is formed by attaching atarget material to a backing plate (a board for attaching a targetthereto). As for the attachment of the target to the backing plate, thetarget may be divided and attached to one backing plate. When the targetis divided, warpage of the target can be relaxed in the attachment ofthe target to the backing plate. In particular, when the thin film isformed over a large substrate, such divided targets can be suitably usedfor a target which is upsized in accordance with the size of the largesubstrate. Needless to say, one target may be attached to one backingplate.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used as a sputtering power source, a DCsputtering method, and a pulsed DC sputtering method in which a bias isapplied in a pulsed manner. An RF sputtering method is mainly used inthe case where an insulating film is formed, and a DC sputtering methodis mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering, and asputtering apparatus used for an ECR sputtering in which plasmagenerated with the use of microwaves is used without using glowdischarge.

Furthermore, as a deposition method by sputtering, there are also areactive sputtering method in which a target substance and a sputteringgas component are chemically reacted with each other during depositionto form a thin compound film thereof, and a bias sputtering in which avoltage is also applied to a substrate during deposition.

Next, a gate insulating layer 102 is formed over the entire surface ofthe gate electrode layer 101. The gate insulating layer 102 is formed toa thickness of greater than or equal to 50 nm and less than or equal to250 nm by a sputtering method, a PCVD method, or the like. The gateinsulating layer 102 is formed to have a single-layer structure or astacked-layer structure using an inorganic insulating film such as asilicon oxide film, a silicon oxynitride film, a silicon nitride oxidefilm, a silicon nitride film, or a tantalum oxide film.

In this embodiment, the gate insulating layer 102 having a thickness of100 nm is formed over the gate electrode layer 101 in the followingmanner: a monosilane gas (SiH₄), nitrous oxide (N₂O), and a rare gas areintroduced into a chamber of a high-density plasma apparatus as sourcegases, and high density plasma is generated under a pressure of 10 Pa to30 Pa. The gate insulating layer 102 is a silicon oxynitride film. Inthis embodiment, the high-density plasma apparatus refers to anapparatus which can realize a plasma density of greater than or equal to1×10¹¹/cm³. For example, plasma is generated by applying a microwavepower of 3 kW to 6 kW so that the insulating film is formed. When theinsulating film is formed, the flow ratio of a monosilane gas (SiH₄) tonitrous oxide (N₂O) which are introduced into the chamber is in therange of 1:10 to 1:200. In addition, as a rare gas which is introducedinto the chamber, helium, argon, krypton, xenon, or the like can beused. In particular, argon, which is inexpensive, is preferably used.

In addition, since the gate insulating layer 102 formed with thehigh-density plasma apparatus has a uniform thickness, the gateinsulating layer 102 has excellent step coverage. Further, by formingthe insulating film using the high-density plasma apparatus, thethickness of the insulating film can be controlled precisely.

The insulating film obtained with the high-density plasma apparatus isgreatly different from an insulating film formed with a conventionalparallel plate PCVD apparatus. The insulating film obtained with thehigh-density plasma apparatus has an etching rate which is lower thanthat of the insulating film formed with the conventional parallel platePCVD apparatus by greater than or equal to 10% or greater than or equalto 20% in the case where the etching rates with the same etchant arecompared to each other. Thus, it can be said that the insulating filmobtained with the high-density plasma apparatus is a dense film.

Next, an oxide semiconductor film (an In—Ga—Zn—O-basednon-single-crystal film) is formed over the gate insulating layer 102.It is effective to form the In—Ga—Zn—O-based non-single-crystal filmwithout exposure to air after the plasma treatment because dust ormoisture can be prevented from being attached to an interface betweenthe gate insulating layer and the semiconductor film. Here, the oxidesemiconductor film is formed in an oxygen atmosphere, in an argonatmosphere, or in an atmosphere including argon and oxygen under theconditions where a target is an oxide semiconductor target containingIn, Ga, and Zn (an In—Ga—Zn—O-based oxide semiconductor target(In₂O₃:Ga₂O₃:ZnO=1:1:1)) with a diameter of 8 inches, the distancebetween the substrate and the target is 170 mm, the pressure is 0.4 Pa,and the direct current (DC) power supply is 0.5 kW. Note that use of apulse direct current (DC) power supply is preferable because this canreduce dust and make the film thickness uniform. The In—Ga—Zn—O-basednon-single-crystal film is formed to have a thickness of 2 nm to 200 nm.As the oxide semiconductor film, an In—Ga—Zn—O-based non-single-crystalfilm with the thickness of 50 nm is formed using the In—Ga—Zn—O-basedoxide semiconductor target by a sputtering method. The oxidesemiconductor film preferably has a thickness of less than or equal to50 nm in order to be kept amorphous. Especially in a channel-etched thinfilm transistor, the oxide semiconductor film is further etched so thata small-thickness region, i.e., a channel formation region, has athickness less than or equal to 30 nm and a small-thickness region ofthe completed thin film transistor has a thickness greater than or equalto 5 nm and less than or equal to 20 nm. In addition, it is preferablethat the channel width of the completed thin film transistor be greaterthan or equal to 0.5 μm and less than or equal to 10 μm.

A target is formed by attaching a target material to a backing plate (aboard for attaching a target thereto) and vacuum packing. In formationof the oxide semiconductor layer, in order to obtain excellent electriccharacteristics of a thin film transistor, it is preferable that thebacking plate including the target material attached thereto be set in asputtering apparatus while being kept away from moisture and the like inthe air as much as possible. It is preferable that the target be keptaway from moisture and the like in the air as much as possible not onlyat the time of setting the target material to the sputtering apparatus,but also during the period up to vacuum-packing, during whichmanufacturing the target, attaching the target materials to the backingplate, and the like are performed.

Next, a second photolithography step is performed to form a resist mask,and then the oxide semiconductor film is etched. For example,unnecessary portions are removed by wet etching using a mixed solutionof phosphoric acid, acetic acid, and nitric acid, so that a first oxidesemiconductor layer 133 is formed (see FIG. 8A). Note that etching hereis not limited to wet etching and dry etching may also be performed.

As an etching gas for dry etching, a gas containing chlorine (achlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused.

Alternatively, a gas containing fluorine (a fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur fluoride (SF₆), nitrogen fluoride(NF₃), or trifluoromethane (CHF₃)); oxygen (O₂); any of these gases towhich a rare gas such as helium (He) or argon (Ar) is added; or the likecan be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the film into a desired shape, the etchingconditions (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on thesubstrate side, the temperature of the electrode on the substrate side,or the like) are adjusted as appropriate.

As an etchant used for wet etching, a mixed solution of phosphoric acid,acetic acid, and nitric acid, or the like can be used. Alternatively,ITO07N (produced by KANTO CHEMICAL CO., INC.) may also be used.

The etchant used in the wet etching is removed by cleaning together withthe material which is etched off. The waste liquid including the etchantand the material etched off may be purified to recycle the materialscontained in the waste liquid. When a material such as indium includedin the oxide semiconductor layer is collected from the waste liquidafter the etching and reused, the resources can be efficiently used andthus the cost can be reduced.

The etching conditions (such as an etchant, etching time, andtemperature) are adjusted as appropriate depending on the material sothat the film can be etched into a desired shape.

The first oxide semiconductor layer 133 is subjected to a first heattreatment for dehydration or dehydrogenation. After the first oxidesemiconductor layer 133 is subjected to the first heat treatment in anoxygen atmosphere, slow cooling in the oxygen atmosphere is performed.

The first heat treatment is performed at a temperature of 650° C. in anoxygen atmosphere for one hour, for example. The slow cooling isperformed from the heating temperature T at which the oxidesemiconductor layer is dehydrated or dehydrogenated to a temperature lowenough to prevent water from coming in again, specifically to atemperature which is more than 100° C. lower than the heatingtemperature T so that a second oxide semiconductor layer 134 is formed.Alternatively, the slow cooling is performed to a temperature lower thana temperature of a second heat treatment which is to be performed laterand then the substrate is taken out of the heat treatment apparatus. Theoxide semiconductor layer is subjected to the heat treatment in theoxygen atmosphere, whereby an impurity such as water contained in theoxide semiconductor layer can be removed and at the same time, thesecond oxide semiconductor layer 134 is placed into a state where oxygenis in excess (see FIG. 8B). The oxide semiconductor layer iscrystallized and changed into a microcrystalline film or apolycrystalline film in some cases depending on the condition of thefirst heat treatment or the material of the oxide semiconductor layer.

Then, a conductive film 132 having a light transmitting property isformed over the second oxide semiconductor layer 134 by a sputteringmethod, a vacuum evaporation method (an electron beam evaporation methodor the like), an arc discharge ion plating method, a spray method, or anink-jet method (see FIG. 8C).

A conductive material having a visible light transmitting property suchas the following materials can be employed as the material of theconductive film 132 having a light transmitting property: anIn—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metal oxide; aSn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide; aSn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; aSn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-basedmetal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide. Thethickness of the conductive film 132 having a light transmittingproperty is selected as appropriate to be within the range of 30 nm to200 nm.

In this embodiment, an example is described in which the first heattreatment for dehydration or dehydrogenation is performed before theconductive film 132 having a light transmitting property is formed;however, the present invention is not particularly limited thereto andthe first heat treatment may be performed after the conductive film 132having a light transmitting property is formed. When the first heattreatment is performed after the conductive film 132 having a lighttransmitting property is formed, the oxide semiconductor layer can bedehydrated or dehydrogenated and at the same time, the conductive film132 having a light transmitting property can have improved crystallinityand low resistance by this heat treatment.

Next, a third photolithography step is performed to form a resist maskand then unnecessary portions are etched away, so that a sourceelectrode layer 105 a, a drain electrode layer 105 b, a capacitorelectrode 135, and a second terminal 122 are formed. Wet etching or dryetching is employed as an etching method at this time. In this etchingstep, an exposed region of the oxide semiconductor layer is partlyetched so that an oxide semiconductor layer 103 having a depression isformed. Therefore, a region of the oxide semiconductor layer 103, whichdoes not overlap with the source electrode layer 105 a and the drainelectrode layer 105 b has a small thickness. In FIG. 8D, the etching forforming the source electrode layer 105 a, the drain electrode layer 105b and the oxide semiconductor layer is performed at a time by dryetching. Accordingly, end portions of the oxide semiconductor layer 103and the source electrode layer 105 a are aligned with each other and arecontinuous while end portions of the oxide semiconductor layer 103 andthe drain electrode layer 105 b are aligned with each other and arecontinuous (these end portions are located above the gate electrodelayer 101).

In the third photolithography step, the second terminal 122 which isformed using the same material as the source electrode layer 105 a orthe drain electrode layer 105 b is left in the terminal portion. Notethat the second terminal 122 is electrically connected to a sourcewiring (a source wiring including the source electrode layer 105 a).

In addition, in the third photolithography step, a storage capacitor isformed by the capacitor wiring 108 and the capacitor electrode 135 whichis formed using the same material as the source electrode layer 105 a orthe drain electrode layer 105 b using the gate electrode layer 102 as adielectric.

Further, with use of a resist mask having regions with a plurality ofthicknesses (typically, two different thicknesses) which is formed usinga multi-tone mask, the number of resist masks can be reduced, whichrealizes a simplified process and lower cost.

Next, the resist mask is removed. When oxygen ashing is performed at thetime of removing the resist mask, oxygen is introduced into an exposedregion of the oxide semiconductor layer 103. Then, a first protectiveinsulating layer 107 serving as a protective insulating layer is formedin contact with part of the oxide semiconductor layer 103. The firstprotective insulating layer 107 is formed using, typically a siliconoxide film, a silicon oxynitride film, an aluminum oxide film, analuminum oxynitride film or the like. Needless to say, the firstprotective insulating layer 107 is an insulating film having a lighttransmitting property.

Then, a heat treatment may be performed after the first protectiveinsulating layer 107 is formed. The heat treatment may be performed at atemperature greater than or equal to 150° C. and less than 350° C. in anoxygen atmosphere or in a nitrogen atmosphere. By the heat treatment,the oxide semiconductor layer 103 is heated while being in contact withthe first protective insulating layer 107; thus, the oxide semiconductorlayer 103 can be made to have a higher resistance, whereby electriccharacteristics of the transistor can be improved and variation inelectric characteristics of the transistor can be reduced. The timing ofthis heat treatment (at a temperature greater than or equal to 150° C.and less than or equal to 350° C., preferably) is not particularlylimited as long as it is after the formation of the first protectiveinsulating layer 107. When this heat treatment also serves as a heattreatment in another step, e.g., a heat treatment in formation of aresin film or a heat treatment for reducing the resistance of aconductive film having a light transmitting property, the number ofsteps can be prevented from increasing.

Through the above steps, a thin film transistor 170 can be completed.

After that, a second protective insulating layer 131 is formed (see FIG.9A). The second protective insulating layer 131 is formed using aninorganic insulating film containing no impurities such as moisture, ahydrogen ion, or OH⁻, which prevents intrusion of these from theoutside; typically, a silicon nitride film, an aluminum nitride film, asilicon nitride oxide film, an aluminum oxynitride film or the like isused. Needless to say, the second protective insulating layer 131 is aninsulating film having a light transmitting property.

Further, the second protective insulating layer 131 is preferably incontact with the gate insulating layer 102 or an insulating film servingas a base which is provided below the second protective insulating layer131, whereby intrusion of an impurity such as moisture, a hydrogen ion,or OH⁻ from a side surface of the substrate is prevented. The abovestructure is effective particularly when a silicon nitride film is usedfor the gate insulating layer 102 or the insulating film serving as abase which is in contact with the second protective insulating layer131.

Next, a fourth photolithography step is performed to form a resist mask.The first protective insulating layer 107, the second protectiveinsulating layer 131, and the gate insulating layer 102 are etched toform a contact hole 125 that reaches the drain electrode layer 105 b. Inaddition, a contact hole 127 that reaches the second terminal 122 and acontact hole 126 that reaches the first terminal 121 are also formed inthe same etching step. A cross-sectional view at this stage isillustrated in FIG. 9B. Note that FIG. 10 is a plan view at this stageand cross-sectional views taken along dotted lines A1-A2 and B1-B2 ofwhich correspond to FIG. 9B. In addition, as illustrated in FIG. 10, acontact hole 124 that reaches the capacitor electrode 135 is also formedin the same etching step.

Next, the resist mask is removed, and then a conductive film having alight transmitting property is formed. The conductive film having alight transmitting property is formed using indium oxide (In₂O₃), indiumoxide-tin oxide alloy (In₂O₃—SnO₂, abbreviated to ITO), or the like by asputtering method, a vacuum evaporation method, or the like. AnAl—Zn—O-based non-single-crystal film including nitrogen, examples ofwhich are an Al—Zn—O—N-based non-single-crystal film, a Zn—O—N-basednon-single-crystal film including nitrogen, and a Sn—Zn—O—N-basednon-single-crystal film including nitrogen, may be used as theconductive film. Note that the relative proportion (atomic %) of zinc inan Al—Zn—O—N-based oxide semiconductor film is less than or equal to 47atomic % and is larger than the relative proportion (atomic %) ofaluminum in the oxide semiconductor film. The relative proportion(atomic %) of aluminum in the oxide semiconductor film is larger thanthe relative proportion (atomic %) of nitrogen in the conductive filmhaving a light transmitting property. Such a material is etched with ahydrochloric acid-based solution. However, since a residue is easilygenerated particularly in etching ITO, indium oxide-zinc oxide alloy(In₂O₃—ZnO) may be used to improve etching processability. Further, whena heat treatment for reducing the resistance of the conductive filmhaving a light transmitting property is performed, the heat treatmentcan serve as a heat treatment for increasing resistance of the oxidesemiconductor layer 103, which results in improvement of electriccharacteristics of the transistor and reduction of variation in theelectric characteristics thereof.

Next, a fifth photolithography step is performed to form a resist mask.Then, an unnecessary portion is etched away, so that a pixel electrodelayer 110 is formed. Note that the pixel electrode layer 110 iselectrically connected to the capacitor electrode 135 through thecontact hole which is formed in the first protective insulating layer107 and the second protective insulating layer 131.

In addition, in the fifth photolithography step, the first terminal 121and the second terminal 122 are covered with the resist mask, andconductive films 128 and 129 having light transmitting properties areleft in the terminal portions. The conductive films 128 and 129 havinglight transmitting properties serve as an electrode or a wiringconnected to an FPC. The conductive film 128 having a light transmittingproperty which is formed over the first terminal 121 is a connectionterminal electrode which functions as an input terminal of the gatewiring. The conductive film 129 having a light transmitting propertywhich is formed over the second terminal 122 is a connection terminalelectrode serving as an input terminal of a source wiring.

Then, the resist mask is removed. A cross-sectional view at this stageis illustrated in FIG. 9C. A plan view at this stage is illustrated inFIG. 11, cross-sectional vies taken along dotted lines A1-A2 and B1-B2of which correspond to FIG. 9C. Although an example is described inwhich the pixel electrode layer 110 overlaps with the channel formationregion and the gate electrode layer 101 of the thin film transistor 170to which the pixel electrode layer 110 is electrically connected, thepresent invention is not particularly limited thereto and the channelformation region of the thin film transistor 170 may overlap with apixel electrode layer of an adjacent pixel, which is not electricallyconnected to the channel formation region. When the conductive filmhaving a light transmitting property, which is the pixel electrode layer110 here, is formed so as to overlap with the channel formation regionof the thin film transistor 170, in a bias-temperature stress test(hereinafter, referred to as a BT test) for examining reliability of athin film transistor, the amount of change in threshold voltage of thethin film transistor 170 between before and after the BT test can bereduced.

FIGS. 12A and 12B are respectively a cross-sectional view and a planview of a gate wiring terminal portion at this stage. FIG. 12A is across-sectional view taken along line E1-E2 of FIG. 12B. In FIG. 12A, aconductive film 155 having a light transmitting property which is formedover a first protective insulating layer 154 and a second protectiveinsulating layer 157 is a connection terminal electrode which functionsas an input terminal. Furthermore, in the terminal portion of FIG. 12A,a first terminal 151 formed of the same material as the gate wiring anda connection electrode layer 153 formed of the same material as thesource wiring overlap with each other with a gate insulating layer 152interposed therebetween, and are electrically connected to each otherthrough the conductive film 155 having a light transmitting property.Note that the portion where the conductive film 128 having a lighttransmitting property is in contact with the first terminal 121 in FIG.9C corresponds to a portion where the conductive film 155 having a lighttransmitting property is in contact with the first terminal 151 in FIG.12A.

FIGS. 12C and 12D are respectively a cross-sectional view and a planview of a source wiring terminal portion which is different from thatillustrated in FIG. 9C. FIG. 12C is a cross-sectional view taken alongline F1-F2 of FIG. 12D. In FIG. 12C, the conductive film 155 having alight transmitting property which is formed over the first protectiveinsulating layer 154 and the second protective insulating layer 157 is aconnection terminal electrode which functions as an input terminal.Furthermore, in the terminal portion of FIG. 12C, an electrode layer 156formed of the same material as the gate wiring is located below a secondterminal 150 electrically connected to the source wiring and overlapswith the second terminal 150 with the gate insulating layer 152therebetween. The electrode layer 156 is not electrically connected tothe second terminal 150, and a capacitor to prevent noise or staticelectricity can be formed if the potential of the electrode layer 156 isset to a potential different from that of the second terminal 150, suchas floating, GND, or 0 V. Further, the second terminal 150 iselectrically connected to the conductive film 155 having a lighttransmitting property with the first protective insulating layer 154 andthe second protective insulating layer 157 therebetween.

A plurality of gate wirings, source wirings, and capacitor wirings areprovided depending on the pixel density. Also in the terminal portion,the first terminal at the same potential as the gate wiring, the secondterminal at the same potential as the source wiring, a third terminal atthe same potential as the capacitor wiring, and the like are eacharranged in plurality. The number of each of the terminals may be anynumber, and the number of the terminals may be determined by apractitioner as appropriate.

Through these five photolithography steps, the storage capacitor and apixel thin film transistor portion including the thin film transistor170 of a bottom-gate staggered thin film transistor can be completedusing the five photomasks. By disposing the thin film transistor and thestorage capacitor in each pixel of a pixel portion in which pixels arearranged in a matrix form, one of substrates for manufacturing an activematrix display device can be obtained. In this specification, such asubstrate is referred to as an active matrix substrate for convenience.

In the case of manufacturing an active matrix liquid crystal displaydevice, an active matrix substrate and a counter substrate provided witha counter electrode are bonded to each other with a liquid crystal layerinterposed therebetween. Note that a common electrode electricallyconnected to the counter electrode on the counter substrate is providedover the active matrix substrate, and a fourth terminal electricallyconnected to the common electrode is provided in the terminal portion.The fourth terminal is provided so that the common electrode is set to afixed potential such as GND or 0 V.

Alternatively, a structure of a storage capacitor is not limited to thatdescribed in this embodiment; for example, instead of providing thecapacitor wiring, the pixel electrode layer may overlap with a gatewiring of an adjacent pixel with the protective insulating layer and thegate insulating layer interposed therebetween, so that a storagecapacitor is formed.

As a method by which a liquid crystal layer is provided between anactive matrix substrate and a counter substrate and sealed, there are aliquid crystal dripping method, a liquid crystal injection method, andthe like. FIGS. 13A to 13C illustrate an example of a liquid crystalpanel in which a liquid crystal layer is provided between the activematrix substrate and the counter substrate, and an FPC 1924 is attached.In a display panel 1908 of FIG. 13A, a first substrate 1920 for which apixel electrode layer is provided and a second substrate 1923 facing thefirst substrate 1920 are attached to each other with a sealant 1922. Thesealant 1922 is formed so as to surround a display portion 1921. Aliquid crystal layer is provided in a region surrounded by the firstsubstrate 1920, the second substrate 1923, and the sealant 1922. In thedisplay panel 1908 illustrated in FIG. 13A, liquid crystal is sealed byutilizing a liquid crystal dripping method, and attaching the substratesunder a reduced pressure. The gap between the pair of substrates ismaintained with a spacer; specifically, a spherical spacer, a columnarspacer, a filler in a sealant, or the like. Note that the spacer may beselected as appropriate depending on a liquid crystal mode (a TN mode, aVA mode, an IPS mode, or the like) for driving the display panel 1908.Note that although the second substrate is not always provided with anelectrode in an IPS mode, in other liquid crystal modes the secondsubstrate is often provided with a counter electrode; and in such acase, when the pair of substrates is attached, connection forelectrically connecting the counter electrode to a terminal electrodeprovided on the first substrate is also carried out.

FIG. 13B illustrates a structural example of a panel manufacturedutilizing a method of sealing liquid crystal which differs from that ofFIG. 13A. Note that in FIG. 13B, portions which are the same as those inFIG. 13A are denoted by the same reference numerals as those used inFIG. 13A. In the display panel illustrated in FIG. 13B, liquid crystalis injected through an inlet for injecting liquid crystal which isformed by a first sealant 1925, using a liquid crystal injecting methodor the like, and then the inlet for injecting liquid crystal is sealedby a second sealant 1926.

FIG. 13C illustrates a structural example of a panel which differs fromthat of FIG. 13A. In FIG. 13C, portions which are the same as those inFIG. 13A are denoted by the same reference numerals as those used inFIG. 13A. In a panel in FIG. 13C, a driver IC 1927 for driving a displayportion is mounted on the first substrate 1920, so that circuits areintegrated.

A desired optical film such as a polarizer, an anti-reflection film, ora color filter may be provided, as appropriate, for the display panelsillustrated in FIGS. 13A to 13C, if necessary.

Block diagrams in FIGS. 14A and 14B shows a structure of an activematrix liquid crystal display device which corresponds to FIG. 13C. InFIG. 14A, a structure of a display portion 1301 which is provided over asubstrate 1300 and a driving portion 1302 which is connected to theoutside of the substrate 1300 is illustrated. The driving portion 1302includes a signal line driver circuit 1303, a scan line driver circuit1304, and the like. In the display portion 1301, a plurality of pixels1305 is provided in matrix.

In FIG. 14A, a scan signal is supplied from the scan line driver circuit1304 to a scan line 1306 through an external connection terminal 1309.In addition, data is supplied from the signal line driver circuit 1303to a signal line 1308 through the external connection terminal 1309. Ascan signal from the scan line 1306 is supplied in such a manner thatthe pixels 1305 are sequentially selected from a first row of the scanline 1306.

Note that in this embodiment, the driving portion 1302 is formed outsideof the substrate 1300 and can be mounted on an FPC (flexible printedcircuit) by a TAB (tape automated bonding) method. Alternatively, thedriving portion 1302 can be mounted on the substrate 1300 by a chip onglass (COG) method.

Note that the driving portion 1302 is formed outside of the substrate1300 and formed using a transistor which uses a single crystalsemiconductor in this embodiment. Therefore, advantages such asimprovement in driving frequency, low power consumption by a reductionin driving voltage, and suppression of variations in output signals canbe obtained in the driving portion 1302. Note also that a signal,voltage, current, or the like is input from the scan line driver circuit1304 and the signal line driver circuit 1303 through the externalconnection terminal 1309.

In FIG. 14A, the scan line driver circuit 1304 is connected to n scanlines 1306 G₁ to G_(n). Considering the case where the minimum imageunit is composed of three pixels of R (red), G (green), and B (blue),the signal line driver circuit 1303 is connected to 3·m signal lines intotal: m signal lines S_(R1) to S_(Rm) corresponding to R; m signallines S_(G1) to S_(Gm) corresponding to G, and m signal lines S_(B1) toS_(Bm) corresponding to B. That is, as illustrated in FIG. 14B, eachcolor element is provided with a signal line and data is supplied fromthe signal line to the pixel corresponding to each color element, sothat the pixels 1305 can express a desired color.

A timing chart of FIG. 15 illustrates scan signals for selecting thescan lines 1306 (e.g., G₁ and G_(n)) in the respective row-selectionperiods (scan period of one row of pixels of the display device) in oneframe period, and a data signal of the signal line 1308 (e.g., SR₁).

Note that block diagrams in FIGS. 14A and 14B, each pixel is providedwith the thin film transistor 170, which is an n-channel transistor.Also in FIG. 15, description is made on the driving of a pixel in thecase of controlling on or off of an n-channel transistor.

In the timing chart of FIG. 15, a row-selection period is 1/(120×n)second on the assumption that one frame period during which an image ofone screen is displayed is set to at least 1/120 second (≈8.3 ms) (morepreferably, 1240 second) so that an afterimage is not visible to anobserver and the number of scan lines is set to n. In the case of adisplay device including 2000 scan lines (considering so-called 4k2kimages with 4096×2160 pixels, 3840×2160 pixels, or the like), arow-selection period is 1/240000 second (≈4.2 μs) if signal delay or thelike due to a wiring is not taken into consideration.

Since the thin film transistor 170 which is arranged in each pixel has alight transmitting property, a high aperture ratio can be realized evenwhen the number of scan lines is increased to, for example, 2000.

In an active matrix liquid crystal display device (for example, a TNtype liquid crystal display device), pixel electrode layers arranged ina matrix form are driven to form a display pattern on a screen.Specifically, voltage is applied between a selected pixel electrodelayer and a counter electrode corresponding to the pixel electrodelayer, and thus, a liquid crystal layer disposed between the pixelelectrode layer and the counter electrode is optically modulated. Thisoptical modulation is recognized as a display pattern by a viewer.

In the TN-type liquid crystal display device, liquid crystal is arrangedin a twisted state at 900 between the pair of the substrates, and theabsorption axis direction of the polarizing element is arranged inapproximately parallel or perpendicular to the rubbing direction. Insuch a TN-type liquid crystal display device, when no voltage is appliedto the pixel electrode layer, incident light from a light source such asa backlight becomes linear polarization in a polarizing element on thelight source side, and this linear polarization is transmitted along thetwist of the liquid crystal layer. In addition, when the transmissionaxis of the other polarizing element is aligned with an azimuth of thelinear polarization, the linear polarization is all emitted to displaywhite (normally white display).

Further, in the case of a full-color liquid crystal display device, acolor filter is provided and color display is performed when no voltageis applied to the pixel electrode layer. Alternatively, when a voltageis applied to the pixel electrode layer, incident light from a lightsource becomes linear polarization in the polarizing element on thelight source side, and the direction of a unit vector showing an averageorientation direction of a liquid crystal molecule axis included in theliquid crystal layer is the approximately perpendicular to the substratesurface. Therefore, the linear polarization is transmitted withoutchanging of an azimuth thereof on the light source side, and the azimuthis aligned with the absorption axis of the other polarizing element, andblack display is obtained.

In this embodiment, an example of the TN-type liquid crystal displaydevice is shown; however, it is not particularly limited, and thepresent invention can be applied to various modes of liquid crystaldisplay devices. For example, as a method for improving viewing anglecharacteristics, the present invention can be applied to a lateralelectric field method (also referred to as IPS) in which an electricfield in the horizontal direction to the main surface of the substrateis applied to the liquid crystal layer. In addition, the presentinvention can be applied to a method in which a vertical alignment filmis used as an alignment film with the use of a nematic liquid crystalmaterial having negative dielectric anisotropy as a liquid crystalmaterial. This method in which the vertical alignment film is used isone of voltage control birefringence (also referred to as ECB) methods,and transmittance is controlled utilizing birefringence of the liquidcrystal molecules.

As a method for improving response speed, response speed of the liquidcrystal layer may be improved so as to response a moving image with theuse of ferroelectric liquid crystal and antiferroelectric liquidcrystal.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which appears just before a cholesteric phase changesinto an isotropic phase while temperature of cholesteric liquid crystalis increased. Since the blue phase appears within an only narrow rangeof temperature, a liquid crystal composition containing a chiral agentat greater than or equal to 5 wt % so as to improve the temperaturerange is used for the liquid crystal layer. The liquid crystalcomposition which includes a liquid crystal showing a blue phase and achiral agent has a short response time of less than or equal to 1 msec,has optical isotropy, which makes the alignment process unneeded, andhas a small viewing angle dependence.

Further, the present invention can be applied to a transmission-typeliquid crystal display device in which an OCB (optical compensatebirefringence) mode is employed. The OCB mode improves response speed ofa liquid crystal layer by the liquid crystal layer between a pair ofsubstrates being made to be in a state referred to as bend alignment. Apretilt angle of the first alignment film in contact with the liquidcrystal layer and a pretilt angle of the second alignment film incontact with the liquid crystal layer are reversed, whereby the bendalignment is made. In this OCB mode, the liquid crystal layer is neededto be transferred from splay alignment that is an initial state to thebend alignment state.

Furthermore, the present invention can be applied to a transmission-typeliquid crystal display device in which a vertical alignment mode isemployed. In the transmission-type liquid crystal display device inwhich the vertical alignment mode is employed, one pixel is set to be aplurality of sub-pixels, and a projection portion is provided in anopposite substrate positioned in a center part of each of sub-pixels,whereby orientation division (multi-domain) of one pixel is performed;accordingly, a driving method for achieving a wide viewing angle may beemployed. This driving method is referred to as sub-pixel driving.

Since the thin film transistor 170 which is arranged in each pixel has alight transmitting property, a high aperture ratio can be realized evenwhen one pixel is divided into a plurality of sub-pixels for sub-pixeldriving in order to realize a wide viewing angle.

In displaying moving images, a liquid crystal display device has aproblem in that a long response time of liquid crystal moleculesthemselves causes afterimages or blurring of moving images. In order toimprove the moving-image characteristics of a liquid crystal displaydevice, a driving method called black insertion is employed in whichblack is displayed on the whole screen every other frame period.

Further, there is another driving method which is so-called double-framerate driving. In the double-frame rate driving, a vertical synchronizingfrequency is set 1.5 times or more, preferably 2 times or more as highas a usual vertical synchronizing frequency, whereby moving imagecharacteristics are improved.

Further alternatively, in order to improve the moving-imagecharacteristics of a liquid crystal display device, a driving method maybe employed, in which a plurality of LEDs (light-emitting diodes) or aplurality of EL light sources is used to form a surface light source asa backlight, and each light source of the surface light source isindependently driven in a pulsed manner in one frame period. As thesurface light source, three or more kinds of LEDs may be used and an LEDemitting white light may be used. Since a plurality of LEDs can becontrolled independently, the light emission timing of LEDs can besynchronized with the timing at which a liquid crystal layer isoptically modulated. According to this driving method, LEDs can bepartly turned off; therefore, an effect of reducing power consumptioncan be obtained particularly in the case of displaying an image having alarge part on which black is displayed on one screen.

By combining these driving methods, the display characteristics of aliquid crystal display device, such as moving-image characteristics, canbe improved as compared to those of conventional liquid crystal displaydevices.

The n-channel transistor disclosed in this specification includes anoxide semiconductor film which is used for a channel formation regionand has excellent dynamic characteristics; thus, it can be combined withthese driving methods.

The use of an oxide semiconductor for a thin film transistor leads to areduction in manufacturing cost. Since moisture and the like which areimpurities are reduced for increasing purity of the oxide semiconductorfilm by a heat treatment for dehydration or dehydrogenation, it is notnecessary to use an ultrapure oxide semiconductor target and a specialsputtering apparatus provided with a deposition chamber whose dew pointtemperature is lowered. Therefore, a semiconductor device including athin film transistor which has favorable electric characteristics andhigh reliability can be manufactured.

The channel formation region in the oxide semiconductor layer is ahigh-resistance region; thus, electric characteristics of the thin filmtransistor are stabilized and increase in off current or the like can beprevented. Therefore, a semiconductor device including a thin filmtransistor which has favorable electric characteristics and highreliability can be provided.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 7

An example of a light-emitting display device will be described as asemiconductor device. As a display element included in a display device,a light-emitting element utilizing electroluminescence is describedhere. Light-emitting elements utilizing electroluminescence areclassified according to whether a light-emitting material is an organiccompound or an inorganic compound. In general, the former is referred toas an organic EL element, and the latter is referred to as an inorganicEL element.

In the organic EL element, by application of a voltage to alight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and thus current flows. Then, the carriers (electrons andholes) recombine, so that the light-emitting organic compound isexcited. The light-emitting organic compound returns to a ground statefrom the excited state, thereby emitting light. Owing to such amechanism, this light-emitting element is referred to as acurrent-excitation light-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. The dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission which utilizes a donorlevel and an acceptor level. The thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade here using an organic EL element as a light-emitting element.

FIG. 16 is a diagram illustrating an example of a pixel structure towhich digital time grayscale driving can be applied, as an example of asemiconductor device.

The structure and operation of a pixel which can be driven by a digitaltime grayscale method will be described. An example is described here inwhich one pixel includes two n-channel transistors each using an oxidesemiconductor layer for a channel formation region.

A pixel 6400 includes a switching transistor 6401, a driving transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driving transistor 6402. The gate of the driving transistor 6402 isconnected to a power source line 6407 through the capacitor 6403, afirst electrode of the driving transistor 6402 is connected to the powersource line 6407, and a second electrode of the driving transistor 6402is connected to a first electrode (pixel electrode layer) of thelight-emitting element 6404. A second electrode of the light-emittingelement 6404 corresponds to a common electrode 6408. The commonelectrode 6408 is electrically connected to a common potential lineprovided over the same substrate.

Note that the second electrode (the common electrode 6408) of thelight-emitting element 6404 is set to a low power source potential. Thelow power source potential is lower than a high power source potentialwhich is supplied to the power source line 6407. For example, GND, 0 V,or the like may be set as the low power source potential. The differencebetween the high power source potential and the low power sourcepotential is applied to the light-emitting element 6404 so that currentflows through the light-emitting element 6404, whereby thelight-emitting element 6404 emits light. Thus, each potential is set sothat the difference between the high power source potential and the lowpower source potential is greater than or equal to a forward thresholdvoltage of the light-emitting element 6404.

When the gate capacitance of the driving transistor 6402 is used as asubstitute for the capacitor 6403, the capacitor 6403 can be omitted.The gate capacitance of the driving transistor 6402 may be formedbetween a channel formation region and a gate electrode.

Here, in the case of using a voltage-input voltage-driving method, avideo signal is input to the gate of the driving transistor 6402 to makethe driving transistor 6402 completely turned on or off. That is, thedriving transistor 6402 operates in a linear region, and thus, a voltagehigher than the voltage of the power source line 6407 is applied to thegate of the driving transistor 6402. Note that a voltage greater than orequal to “power source line voltage+V_(th) of the driving transistor6402” is applied to the signal line 6405.

In the case of using an analog grayscale method instead of the digitaltime grayscale method, the same pixel structure as in FIG. 16 can beemployed by inputting signals in a different way.

In the case of using the analog grayscale driving method, a voltagegreater than or equal to “forward voltage of the light-emitting element6404+V_(th) of the driving transistor 6402” is applied to the gate ofthe driving transistor 6402. The forward voltage of the light-emittingelement 6404 refers to a voltage to obtain a desired luminance, andincludes at least a forward threshold voltage. By inputting a videosignal to enable the driving transistor 6402 to operate in a saturationregion, current can flow through the light-emitting element 6404. Inorder that the driving transistor 6402 can operate in the saturationregion, the potential of the power source line 6407 is higher than agate potential of the driving transistor 6402. With the analog videosignal, current in accordance with the video signal flows through thelight-emitting element 6404, and the analog grayscale driving method canbe performed.

Note that the pixel structure is not limited to that illustrated in FIG.16. For example, the pixel in FIG. 16 can further include a switch, aresistor, a capacitor, a transistor, a logic circuit, or the like.

Next, structures of the light-emitting element are described withreference to FIGS. 17A to 17C. Here, a cross-sectional structure of apixel is described by taking an n-channel driving TFT as an example.Driving TFTs 7001, 7011, and 7021 used in semiconductor devicesillustrated in FIGS. 17A, 17B, and 17C, respectively, can be formed in amanner similar to that of the thin film transistor described inEmbodiment 1 and are highly reliable thin film transistors eachincluding an oxide semiconductor layer. Alternatively, the thin filmtransistors described in Embodiment 2 or 3 can be used as the drivingTFTs 7001, 7011, and 7021.

In order to extract light emitted from the light-emitting element, atleast one of an anode and a cathode is required to transmit light. Athin film transistor and a light-emitting element are formed over asubstrate. A light-emitting element can have a top emission structure inwhich light is extracted through the surface opposite to the substrate,a bottom emission structure in which light is extracted through thesurface on the substrate side, or a dual emission structure in whichlight is extracted through the surface opposite to the substrate and thesurface on the substrate side. The pixel structure can be applied to alight-emitting element having any of these emission structures.

A light-emitting element having a top emission structure is describedwith reference to FIG. 17A.

FIG. 17A is a cross-sectional view of a pixel in the case where thedriving TFT 7001 is an n-type TFT and light is emitted from alight-emitting element 7002 to an anode 7005 side. In FIG. 17A, acathode 7003 of the light-emitting element 7002 is electricallyconnected to the driving TFT 7001, and a light-emitting layer 7004 andthe anode 7005 are stacked in this order over the cathode 7003. Further,the driving TFT 7001 is covered with a protective insulating layer 7006which is a silicon nitride film, an aluminum nitride film or the likeand further covered with a planarization insulating film 7007. Thecathode 7003 can be formed using a variety of conductive materials aslong as they have a low work function and reflect light. For example,Ca, Al, MgAg, AlLi, or the like is preferably used. The light-emittinglayer 7004 may be formed using a single layer or a plurality of layersstacked. When the light-emitting layer 7004 is formed using a pluralityof layers, the light-emitting layer 7004 is formed by stacking anelectron-injection layer, an electron-transport layer, a light-emittinglayer, a hole-transport layer, and a hole-injection layer in this orderover the cathode 7003. However, it is not necessary to form all of theselayers. The anode 7005 is formed using a conductive film having a lighttransmitting property such as a film of indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, orindium tin oxide to which silicon oxide is added.

The cathode 7003 is insulated from a cathode 7008 of an adjacent pixelby a partition wall 7009. The cathode 7008 of the adjacent pixeloverlaps with an oxide semiconductor layer and a gate insulating layerof the driving TFT 7001. In the case where a bias-temperature stresstest (hereinafter, referred to as a BT test) for examining reliabilityof a thin film transistor is carried out, by forming the cathode 7008 ofthe adjacent pixel which overlaps with a channel formation region of thedriving TFT 7001, the amount of change in threshold voltage of thedriving TFT 7001 between before and after the BT test can be reduced.

A region where the light-emitting layer 7004 is sandwiched between thecathode 7003 and the anode 7005 corresponds to the light-emittingelement 7002. In the case of the pixel illustrated in FIG. 17A, light isemitted from the light-emitting element 7002 to the anode 7005 side asindicated by arrows.

Next, a light-emitting element having a bottom emission structure isdescribed with reference to FIG. 17B. FIG. 17B is a cross-sectional viewof a pixel in the case where the driving TFT 7011 is an n-type TFT andlight is emitted from a light-emitting element 7012 to a cathode 7013side. In FIG. 17B, the cathode 7013 of the light-emitting element 7012is formed over a conductive film 7017 having a light transmittingproperty which is electrically connected to the driving TFT 7011, and alight-emitting layer 7014 and an anode 7015 are stacked in this orderover the cathode 7013. A light-blocking film 7016 for reflecting orblocking light may be formed so as to cover the anode 7015 when theanode 7015 has a light transmitting property. For the cathode 7013,various materials can be used as in the case of FIG. 17A as long as thecathode 7013 is formed using a conductive material having a low workfunction. The cathode 7013 is formed to have a thickness that enablestransmission of light (preferably, approximately 5 nm to 30 nm). Forexample, an aluminum film with a thickness of 20 nm can be used as thecathode 7013. Similar to the case of FIG. 17A, the light-emitting layer7014 may be formed using either a single layer or a plurality of layersstacked. The anode 7015 is not required to transmit light, but can beformed using a conductive material having a light transmitting propertyas in the case of FIG. 17A. As the light-blocking film 7016, a metal orthe like that reflects light can be used, for example; however, it isnot limited to a metal film. For example, a resin or the like to whichblack pigments are added can also be used.

The cathode 7013 is insulated from a cathode 7018 of an adjacent pixelby a partition wall 7019. The cathode 7018 of the adjacent pixeloverlaps with an oxide semiconductor layer and a gate insulating layerof the driving TFT 7011. In the case where a bias-temperature stresstest (hereinafter, referred to as a BT test) for examining reliabilityof a thin film transistor is carried out, by forming the cathode 7018 ofthe adjacent pixel which overlaps with a channel formation region of thedriving TFT 7011, the amount of change in threshold voltage of thedriving TFT 7011 between before and after the BT test can be reduced.

A region where the light-emitting layer 7014 is sandwiched between thecathode 7013 and the anode 7015 corresponds to the light-emittingelement 7012. In the case of the pixel illustrated in FIG. 17B, light isemitted from the light-emitting element 7012 to the cathode 7013 side asindicated by an arrow.

Further, since the driving TFT 7011 has a light transmitting property,light emitted from a light-emitting element of the pixel adjacent to thelight-emitting element 7012 is emitted to the cathode 7013 side throughthe driving TFT 7011 as indicated by an arrow.

Next, a light-emitting element having a dual emission structure isdescribed with reference to FIG. 17C. In FIG. 17C, a cathode 7023 of alight-emitting element 7022 is formed over a conductive film 7027 havinga light transmitting property which is electrically connected to thedriving TFT 7021, and a light-emitting layer 7024 and an anode 7025 arestacked in this order over the cathode 7023. As in the case of FIG. 17A,the cathode 7023 can be formed using any of a variety of materials aslong as the cathode 7023 is formed using a conductive material having alow work function. The cathode 7023 is formed to have a thickness thatenables transmission of light. For example, an Al film having athickness of 20 nm can be used as the cathode 7023. The light-emittinglayer 7024 may be formed using a single layer or a plurality of layersstacked as in the case of FIG. 17A. As in the case of FIG. 17A, theanode 7025 can be formed using a conductive material having a lighttransmitting property.

The cathode 7023 is insulated from a cathode 7028 of an adjacent pixelby a partition wall 7029. The cathode 7028 of the adjacent pixeloverlaps with an oxide semiconductor layer and a gate insulating layerof the driving TFT 7021. In the case where a bias-temperature stresstest (hereinafter, referred to as a BT test) for examining reliabilityof a thin film transistor is carried out, by forming the cathode 7028 ofthe adjacent pixel which overlaps with a channel formation region of thedriving TFT 7021, the amount of change in threshold voltage of thedriving TFT 7021 between before and after the BT test can be reduced.

A region where the cathode 7023, the light-emitting layer 7024, and theanode 7025 overlap with one another corresponds to the light-emittingelement 7022. In the case of the pixel illustrated in FIG. 17C, light isemitted from the light-emitting element 7022 to both the anode 7025 sideand the cathode 7023 side as indicated by arrows.

Further, since the driving TFT 7021 has a light transmitting property,light emitted from a light-emitting element of the pixel adjacent to thelight-emitting element 7022 is emitted to a cathode 7023 side throughthe driving TFT 7021 as indicated by an arrow.

Although an example in which the cathode which is a pixel electrodelayer overlaps with the channel formation region of the TFT of theadjacent pixel is described in this embodiment, the present invention isnot particularly limited to this example, and a structure in which thecathode overlaps with the channel formation region of the TFT to whichthe cathode is electrically connected may be employed.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element can also be provided as alight-emitting element.

Note that the example is described in which a thin film transistor (adriving TFT) which controls the driving of a light-emitting element iselectrically connected to the light-emitting element; however, astructure may be employed in which a TFT for current control isconnected between the driving TFT and the light-emitting element.

Note that the structure of the semiconductor device is not limited tothose illustrated in FIGS. 17A to 17C and can be modified in variousways based on techniques disclosed in this specification.

Through the above steps, a highly reliable light-emitting display device(display panel) can be manufactured as a semiconductor device.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 8

A semiconductor device which has a thin film transistor manufactured bythe process described in any of Embodiments 1 to 7 disclosed in thisspecification can be applied to a variety of electronic appliances(including amusement machines).

Examples of electronic appliances include a television set (alsoreferred to as a television or a television receiver), a monitor of acomputer or the like, a camera such as a digital camera or a digitalvideo camera, a digital photo frame, a mobile phone set (also referredto as a mobile phone or a mobile phone device), a portable game console,a portable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.

FIG. 18A illustrates an example of a mobile phone 1000. The mobile phone1000 includes a display portion 1002 incorporated in a housing 1001,operation buttons 1003, an external connection port 1004, a speaker1005, a microphone 1006 and the like.

When the display portion 1002 of the mobile phone 1000 illustrated inFIG. 18A is touched with a finger or the like, data can be input intothe mobile phone 1000. Furthermore, operations such as making calls andcomposing mails can be performed by touching the display portion 1002with a finger or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in a case of making a call or composing a mail, a textinput mode mainly for inputting text is selected for the display portion1002 so that text displayed on a screen can be input. In that case, itis preferable to display a keyboard or number buttons on almost all areaof the screen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 1000, display on the screen of the display portion 1002 canbe automatically switched by determining the installation direction ofthe mobile phone 1000 (whether the mobile phone 1000 is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 1002 oroperating the operation buttons 1003 of the housing 1001. Alternatively,the screen modes can be switched depending on the kind of the imagedisplayed on the display portion 1002. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 1002 can function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken when thedisplay portion 1002 is touched with a palm or a finger, wherebypersonal identification can be performed. Further, by providing abacklight which emits near-infrared light or a sensing light sourcewhich emits near-infrared light for the display portion, an image of afinger vein, a palm vein, or the like can be taken.

A plurality of thin film transistors which is described in Embodiment 1is arranged in the display portion 1002. Since the thin film transistorsand wirings have light transmitting properties, they do not blockincident light in the case of providing an optical sensor for thedisplay portion 1002 and thus are effective. In addition, also in thecase of providing a backlight which emits near-infrared light or asensing light source which emits near-infrared light for the displayportion, the thin film transistors and the wirings do not block lightand thus are effective.

FIG. 18B also illustrates an example of a mobile phone. A portableinformation terminal whose example is illustrated in FIG. 18B can have aplurality of functions. For example, in addition to a telephonefunction, such a portable information terminal can have a function ofprocessing a variety of pieces of data by incorporating a computer.

The portable information terminal illustrated in FIG. 18B has a housing1800 and a housing 1801. The housing 1800 includes a display panel 1802,a speaker 1803, a microphone 1804, a pointing device 1806, a camera lens1807, an external connection terminal 1808, and the like. The housing1801 includes a keyboard 1810, an external memory slot 1811, and thelike. In addition, an antenna is incorporated in the housing 1801.

The display panel 1802 is provided with a touch panel. A plurality ofoperation keys 1805 which is displayed as images is illustrated bydashed lines in FIG. 18B.

Further, in addition to the above structure, a contactless IC chip, asmall memory device, or the like may be incorporated.

The display device of the present invention can be used for the displaypanel 1802 and the direction of display is changed appropriatelydepending on an application mode. Further, the display device isprovided with the camera lens 1807 on the same surface as the displaypanel 1802, and thus it can be used as a video phone. The speaker 1803and the microphone 1804 can be used for videophone calls, recording, andplaying sound, etc. as well as voice calls. Moreover, the housings 1800and 1801 in a state where they are developed as illustrated in FIG. 18Bcan shift so that one is lapped over the other by sliding; therefore,the size of the portable information terminal can be reduced, whichmakes the portable information terminal suitable for being carried.

The external connection terminal 1808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a storagemedium can be inserted into the external memory slot 1811 so that alarge amount of data can be stored and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 19A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. Images can be displayed on the display portion 9603. Here, thehousing 9601 is supported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with an operation key 9609 of the remote controller9610 so that an image displayed on the display portion 9603 can becontrolled. Furthermore, the remote controller 9610 may be provided witha display portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the receiver, a general television broadcast can bereceived. Furthermore, when the television set 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver, between receivers, or the like) datacommunication can be performed.

Since a plurality of thin film transistors having light transmittingproperties which are described in Embodiment 1 is arranged in thedisplay portion 9603, an aperture ratio can be high also in the case ofrealizing an image with high definition by increasing the number of scanlines, for example, to 2000 (considering so-called 4k2k images with4096×2160 pixels, 3840×2160 pixels, or the like). However, when the sizeof the display portion 9603 is 60 inches, 120 inches, or the like, whichexceeds 10 inches, there is a concern that the wiring resistance of awiring having a light transmitting property becomes problematic;therefore, a scan line or source line is preferably provided with alow-resistance metal wiring as an auxiliary wiring.

FIG. 19B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. Various images can be displayed on thedisplay portion 9703. For example, the display portion 9703 can displaydata of an image shot by a digital camera or the like to function as anormal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalthat can be connected to various cables such as a USB cable, or thelike), a recording medium insertion portion, and the like. Although theymay be provided on the same surface as the display portion, it ispreferable to provide them on the side surface or the back surface forthe design of the digital photo frame 9700. For example, a memorystoring data of an image shot by a digital camera is inserted into therecording medium insertion portion of the digital photo frame, wherebythe image data can be transferred and displayed on the display portion9703.

The digital photo frame 9700 may have a configuration capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, desired image data can be transferred to be displayed.

FIG. 20 illustrates a portable amusement machine including two housings:a housing 9881 and a housing 9891. The housings 9881 and 9891 areconnected with a connection portion 9893 so as to be opened and closed.A display portion 9882 and a display portion 9883 are incorporated inthe housing 9881 and the housing 9891, respectively. In addition, theportable amusement machine illustrated in FIG. 20 includes a speakerportion 9884, a recording medium insertion portion 9886, an LED lamp9890, an input means (an operation key 9885, a connection terminal 9887,a sensor 9888 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), or a microphone 9889), and thelike. It is needless to say that the structure of the portable amusementmachine is not limited to the above and other structures provided withat least a semiconductor device disclosed in this specification can beemployed. The portable amusement machine may include other accessoryequipment as appropriate. The portable amusement machine illustrated inFIG. 20 has a function of reading a program or data stored in arecording medium to display it on the display portion, and a function ofsharing information with another portable amusement machine by wirelesscommunication. The portable amusement machine illustrated in FIG. 20 canhave various functions without limitation to the above.

As described above, the thin film transistor having a light transmittingproperty can be arranged in a display portion or a display panel of avariety of electronic appliances such as the above ones. A highlyreliable electronic appliance having a display portion with a highaperture ratio can be provided by using the thin film transistor havinga light transmitting property as a switching element of the displaypanel.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

This application is based on Japanese Patent Application serial no.2009-164265 filed with Japan Patent Office on Jul. 10, 2009, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a pixel, the pixel comprising: astorage capacitor; and a transistor, the transistor comprising: a gateelectrode layer over a substrate with an insulating surface; a gateinsulating layer over the gate electrode layer; an oxide semiconductorlayer over the gate insulating layer; a source electrode layer having aregion in contact with a top surface of the oxide semiconductor layer; adrain electrode layer having a region in contact with the top surface ofthe oxide semiconductor layer; and an insulating layer over the sourceelectrode layer and the drain electrode layer; a pixel electrode layerover the insulating layer, wherein the pixel electrode layer is incontact with a top surface of one of the source electrode layer and thedrain electrode layer via an opening in the insulating layer, whereinthe pixel electrode has a region overlapping with an electrodecomprising a metal oxide via the insulating layer, and wherein theelectrode functions as one electrode of the storage capacitor.
 2. Asemiconductor device comprising: a pixel, the pixel comprising: astorage capacitor; and a transistor, the transistor comprising: a gateelectrode layer over a substrate with an insulating surface; a gateinsulating layer over the gate electrode layer; an oxide semiconductorlayer over the gate insulating layer; a source electrode layer having aregion in contact with a top surface of the oxide semiconductor layer; adrain electrode layer having a region in contact with the top surface ofthe oxide semiconductor layer; and an insulating layer over the sourceelectrode layer and the drain electrode layer; a pixel electrode layerover the insulating layer; and a light-emitting layer over the pixelelectrode layer, wherein the pixel electrode layer is in contact with atop surface of one of the source electrode layer and the drain electrodelayer via an opening in the insulating layer, wherein the pixelelectrode has a region overlapping with an electrode comprising a metaloxide via the insulating layer, wherein the electrode functions as oneelectrode of the storage capacitor, and wherein light emitted from thelight-emitting layer is emitted to a transistor side.
 3. A semiconductordevice comprising: a pixel, the pixel comprising: a storage capacitor;and a transistor, the transistor comprising: a gate electrode layer overand in contact with a substrate with an insulating surface; a gateinsulating layer over the gate electrode layer; a first oxidesemiconductor layer over the gate insulating layer; a second oxidesemiconductor layer over the first oxide semiconductor layer; a sourceelectrode layer having a region in contact with a top surface of thesecond oxide semiconductor layer; a drain electrode layer having aregion in contact with the top surface of the second oxide semiconductorlayer; and an insulating layer over the source electrode layer and thedrain electrode layer; a pixel electrode layer over the insulatinglayer; and a light-emitting layer over the pixel electrode layer,wherein the pixel electrode layer is in contact with a top surface ofone of the source electrode layer and the drain electrode layer via anopening in the insulating layer, wherein the pixel electrode has aregion overlapping with an electrode comprising a metal oxide via theinsulating layer, wherein the electrode functions as one electrode ofthe storage capacitor, and wherein light emitted from the light-emittinglayer is emitted to a transistor side.
 4. The semiconductor deviceaccording to claim 1, wherein the oxide semiconductor layer comprisesIn, Ga, and Zn.
 5. The semiconductor device according to claim 2,wherein the oxide semiconductor layer comprises In, Ga, and Zn.
 6. Thesemiconductor device according to claim 3, wherein the first oxidesemiconductor layer comprises In, Ga, and Zn.
 7. The semiconductordevice according to claim 1, wherein each of the oxide semiconductorlayer and the metal oxide comprises In, Ga, and Zn.
 8. The semiconductordevice according to claim 2, wherein each of the oxide semiconductorlayer and the metal oxide comprises In, Ga, and Zn.
 9. The semiconductordevice according to claim 3, wherein each of the first oxidesemiconductor layer and the metal oxide comprises In, Ga, and Zn. 10.(canceled)
 11. The semiconductor device according to claim 1, whereinthe oxide semiconductor layer comprises a crystalline region.
 12. Thesemiconductor device according to claim 2, wherein the oxidesemiconductor layer comprises a crystalline region.
 13. Thesemiconductor device according to claim 3, wherein the first oxidesemiconductor layer comprises a crystalline region.